Here's another example of an odd test failure apparently caused by test_pydoc side effects, this using current default tip (same results with the current cherry pick for 3.4.0rc2):

./bin/python3.4 -m test -w -uall,-largefile test_pydoc test_unicode
[1/2] test_pydoc
Warning -- warnings.filters was modified by test_pydoc
Warning -- logging._handlerList was modified by test_pydoc
[2/2] test_unicode
test test_unicode failed -- Traceback (most recent call last):
  File "/py/dev/3x/root/fwn/Library/Frameworks/pytest_10.9.framework/Versions/3.4/lib/python3.4/test/test_unicode.py", line 1072, in test_formatting
    string_tests.MixinStrUnicodeUserStringTest.test_formatting(self)
  File "/py/dev/3x/root/fwn/Library/Frameworks/pytest_10.9.framework/Versions/3.4/lib/python3.4/test/string_tests.py", line 1181, in test_formatting
    with self.assertWarns(DeprecationWarning):
  File "/py/dev/3x/root/fwn/Library/Frameworks/pytest_10.9.framework/Versions/3.4/lib/python3.4/unittest/case.py", line 202, in __enter__
    for v in sys.modules.values():
RuntimeError: dictionary changed size during iteration

1 test failed:
    test_unicode
1 test altered the execution environment:
    test_pydoc
Re-running failed tests in verbose mode
Re-running test 'test_unicode' in verbose mode
test_formatter_field_name_split (test.test_unicode.StringModuleTest) ... ok
test_formatter_parser (test.test_unicode.StringModuleTest) ... ok
test___contains__ (test.test_unicode.UnicodeTest) ... ok
test_additional_rsplit (test.test_unicode.UnicodeTest) ... ok
test_additional_split (test.test_unicode.UnicodeTest) ... ok
test_ascii (test.test_unicode.UnicodeTest) ... ok
test_aswidechar (test.test_unicode.UnicodeTest) ... ok
test_aswidecharstring (test.test_unicode.UnicodeTest) ... ok
test_bug1001011 (test.test_unicode.UnicodeTest) ... ok
test_bytes_comparison (test.test_unicode.UnicodeTest) ... ok
test_capitalize (test.test_unicode.UnicodeTest) ... ok
test_casefold (test.test_unicode.UnicodeTest) ... ok
test_center (test.test_unicode.UnicodeTest) ... ok
test_codecs (test.test_unicode.UnicodeTest) ... ok
test_codecs_charmap (test.test_unicode.UnicodeTest) ... ok
test_codecs_errors (test.test_unicode.UnicodeTest) ... ok
test_codecs_idna (test.test_unicode.UnicodeTest) ... ok
test_codecs_utf7 (test.test_unicode.UnicodeTest) ... ok
test_codecs_utf8 (test.test_unicode.UnicodeTest) ... ok
test_compare (test.test_unicode.UnicodeTest) ... ok
test_comparison (test.test_unicode.UnicodeTest) ... ok
test_concatenation (test.test_unicode.UnicodeTest) ... ok
test_constructor (test.test_unicode.UnicodeTest) ... ok
test_constructor_defaults (test.test_unicode.UnicodeTest)
Check the constructor argument defaults. ... ok
test_constructor_keyword_args (test.test_unicode.UnicodeTest)
Pass various keyword argument combinations to the constructor. ... ok
test_contains (test.test_unicode.UnicodeTest) ... ok
test_conversion (test.test_unicode.UnicodeTest) ... ok
test_count (test.test_unicode.UnicodeTest) ... ok
test_encode_decimal (test.test_unicode.UnicodeTest) ... ok
test_endswith (test.test_unicode.UnicodeTest) ... ok
test_expandtabs (test.test_unicode.UnicodeTest) ... ok
test_expandtabs_optimization (test.test_unicode.UnicodeTest) ... ok
test_expandtabs_overflows_gracefully (test.test_unicode.UnicodeTest) ... skipped 'only applies to 32-bit platforms'
test_extended_getslice (test.test_unicode.UnicodeTest) ... ok
test_find (test.test_unicode.UnicodeTest) ... ok
test_find_etc_raise_correct_error_messages (test.test_unicode.UnicodeTest) ... ok
test_floatformatting (test.test_unicode.UnicodeTest) ... ok
test_format (test.test_unicode.UnicodeTest) ... ok
test_format_auto_numbering (test.test_unicode.UnicodeTest) ... ok
test_format_float (test.test_unicode.UnicodeTest) ... ok
test_format_huge_item_number (test.test_unicode.UnicodeTest) ... ok
test_format_huge_precision (test.test_unicode.UnicodeTest) ... ok
test_format_huge_width (test.test_unicode.UnicodeTest) ... ok
test_format_map (test.test_unicode.UnicodeTest) ... ok
test_format_subclass (test.test_unicode.UnicodeTest) ... ok
test_formatting (test.test_unicode.UnicodeTest) ... ERROR
test_formatting_c_limits (test.test_unicode.UnicodeTest) ... ok
test_formatting_huge_precision (test.test_unicode.UnicodeTest) ... ok
test_formatting_huge_precision_c_limits (test.test_unicode.UnicodeTest) ... ok
test_formatting_huge_width (test.test_unicode.UnicodeTest) ... ok
test_formatting_with_enum (test.test_unicode.UnicodeTest) ... ok
test_from_format (test.test_unicode.UnicodeTest) ... ok
test_getnewargs (test.test_unicode.UnicodeTest) ... ok
test_hash (test.test_unicode.UnicodeTest) ... ok
test_index (test.test_unicode.UnicodeTest) ... ok
test_inplace_rewrites (test.test_unicode.UnicodeTest) ... ok
test_invalid_cb_for_2bytes_seq (test.test_unicode.UnicodeTest) ... ok
test_invalid_cb_for_3bytes_seq (test.test_unicode.UnicodeTest) ... ok
test_invalid_cb_for_4bytes_seq (test.test_unicode.UnicodeTest) ... ok
test_invalid_start_byte (test.test_unicode.UnicodeTest) ... ok
test_isalnum (test.test_unicode.UnicodeTest) ... ok
test_isalpha (test.test_unicode.UnicodeTest) ... ok
test_isdecimal (test.test_unicode.UnicodeTest) ... ok
test_isdigit (test.test_unicode.UnicodeTest) ... ok
test_isidentifier (test.test_unicode.UnicodeTest) ... ok
test_islower (test.test_unicode.UnicodeTest) ... ok
test_isnumeric (test.test_unicode.UnicodeTest) ... ok
test_isprintable (test.test_unicode.UnicodeTest) ... ok
test_isspace (test.test_unicode.UnicodeTest) ... ok
test_issue18183 (test.test_unicode.UnicodeTest) ... ok
test_issue8271 (test.test_unicode.UnicodeTest) ... ok
test_istitle (test.test_unicode.UnicodeTest) ... ok
test_isupper (test.test_unicode.UnicodeTest) ... ok
test_iterators (test.test_unicode.UnicodeTest) ... ok
test_join (test.test_unicode.UnicodeTest) ... ok
test_literals (test.test_unicode.UnicodeTest) ... ok
test_ljust (test.test_unicode.UnicodeTest) ... ok
test_lower (test.test_unicode.UnicodeTest) ... ok
test_maketrans_translate (test.test_unicode.UnicodeTest) ... ok
test_mul (test.test_unicode.UnicodeTest) ... ok
test_none_arguments (test.test_unicode.UnicodeTest) ... ok
test_partition (test.test_unicode.UnicodeTest) ... ok
test_printable_repr (test.test_unicode.UnicodeTest) ... ok
test_printing (test.test_unicode.UnicodeTest) ... ok
test_raiseMemError (test.test_unicode.UnicodeTest) ... ok
test_replace (test.test_unicode.UnicodeTest) ... ok
test_replace_id (test.test_unicode.UnicodeTest) ... ok
test_replace_overflow (test.test_unicode.UnicodeTest) ... skipped 'only applies to 32-bit platforms'
test_repr (test.test_unicode.UnicodeTest) ... ok
test_resize (test.test_unicode.UnicodeTest) ... ok
test_rfind (test.test_unicode.UnicodeTest) ... ok
test_rindex (test.test_unicode.UnicodeTest) ... ok
test_rjust (test.test_unicode.UnicodeTest) ... ok
test_rpartition (test.test_unicode.UnicodeTest) ... ok
test_rsplit (test.test_unicode.UnicodeTest) ... ok
test_slice (test.test_unicode.UnicodeTest) ... ok
test_split (test.test_unicode.UnicodeTest) ... ok
test_splitlines (test.test_unicode.UnicodeTest) ... ok
test_startswith (test.test_unicode.UnicodeTest) ... ok
test_startswith_endswith_errors (test.test_unicode.UnicodeTest) ... ok
test_strip (test.test_unicode.UnicodeTest) ... ok
test_subclass_add (test.test_unicode.UnicodeTest) ... ok
test_subscript (test.test_unicode.UnicodeTest) ... ok
test_surrogates (test.test_unicode.UnicodeTest) ... ok
test_swapcase (test.test_unicode.UnicodeTest) ... ok
test_title (test.test_unicode.UnicodeTest) ... ok
test_transform_decimal (test.test_unicode.UnicodeTest) ... ok
test_ucs4 (test.test_unicode.UnicodeTest) ... ok
test_unexpected_end_of_data (test.test_unicode.UnicodeTest) ... ok
test_unicode_repr (test.test_unicode.UnicodeTest) ... ok
test_upper (test.test_unicode.UnicodeTest) ... ok
test_utf8_decode_invalid_sequences (test.test_unicode.UnicodeTest) ... ok
test_utf8_decode_valid_sequences (test.test_unicode.UnicodeTest) ... ok
test_zfill (test.test_unicode.UnicodeTest) ... ok

======================================================================
ERROR: test_formatting (test.test_unicode.UnicodeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/py/dev/3x/root/fwn/Library/Frameworks/pytest_10.9.framework/Versions/3.4/lib/python3.4/test/test_unicode.py", line 1072, in test_formatting
    string_tests.MixinStrUnicodeUserStringTest.test_formatting(self)
  File "/py/dev/3x/root/fwn/Library/Frameworks/pytest_10.9.framework/Versions/3.4/lib/python3.4/test/string_tests.py", line 1181, in test_formatting
    with self.assertWarns(DeprecationWarning):
  File "/py/dev/3x/root/fwn/Library/Frameworks/pytest_10.9.framework/Versions/3.4/lib/python3.4/unittest/case.py", line 202, in __enter__
    for v in sys.modules.values():
RuntimeError: dictionary changed size during iteration

----------------------------------------------------------------------
Ran 114 tests in 1.122s

FAILED (errors=1, skipped=2)
test test_unicode failed