For clarity, this is what i get in the test_exceptions test when i've applied the attached diff.

$ ./python -m test -v test_exceptions
== CPython 3.7.0a0 (default:8e311f109b22, Dec 24 2016, 12:26:43) [GCC 5.4.0 20160609]
==   Linux-4.4.0-53-generic-x86_64-with-debian-stretch-sid little-endian
==   hash algorithm: siphash24 64bit
==  cwd: /home/irina/patrik/cpython/build/test_python_19104
==  encodings: locale=UTF-8, FS=utf-8
Testing with flags: sys.flags(debug=0, inspect=0, interactive=0, optimize=0, dont_write_bytecode=0, no_user_site=0, no_site=0, ignore_environment=0, verbose=0, bytes_warning=0, quiet=0, hash_randomization=1, isolated=0)
Run tests sequentially
0:00:00 [1/1] test_exceptions
testAttributes (test.test_exceptions.ExceptionTests) ... ok
testChainingAttrs (test.test_exceptions.ExceptionTests) ... ok
testChainingDescriptors (test.test_exceptions.ExceptionTests) ... ok
testExceptionCleanupNames (test.test_exceptions.ExceptionTests) ... ok
testExceptionCleanupState (test.test_exceptions.ExceptionTests) ... ok
testInfiniteRecursion (test.test_exceptions.ExceptionTests) ... ok
testInvalidAttrs (test.test_exceptions.ExceptionTests) ... ok
testInvalidTraceback (test.test_exceptions.ExceptionTests) ... ok
testKeywordArgs (test.test_exceptions.ExceptionTests) ... ok
testNoneClearsTracebackAttr (test.test_exceptions.ExceptionTests) ... ok
testRaising (test.test_exceptions.ExceptionTests) ... ok
testSettingException (test.test_exceptions.ExceptionTests) ... ok
testSyntaxErrorMessage (test.test_exceptions.ExceptionTests) ... ok
testSyntaxErrorOffset (test.test_exceptions.ExceptionTests) ... ok
testWithTraceback (test.test_exceptions.ExceptionTests) ... ok
test_3114 (test.test_exceptions.ExceptionTests) ... ok
test_MemoryError (test.test_exceptions.ExceptionTests) ... ok
test_WindowsError (test.test_exceptions.ExceptionTests) ... ok
test_badisinstance (test.test_exceptions.ExceptionTests) ... ok
test_errno_ENOTDIR (test.test_exceptions.ExceptionTests) ... ok
test_exception_target_in_nested_scope (test.test_exceptions.ExceptionTests) ... ok
test_exception_with_doc (test.test_exceptions.ExceptionTests) ... ok
test_generator_close_cleanup_exc_state (test.test_exceptions.ExceptionTests) ... ok
test_generator_del_cleanup_exc_state (test.test_exceptions.ExceptionTests) ... ok
test_generator_doesnt_retain_old_exc (test.test_exceptions.ExceptionTests) ... ok
test_generator_finalizing_and_exc_info (test.test_exceptions.ExceptionTests) ... ok
test_generator_leaking (test.test_exceptions.ExceptionTests) ... ok
test_generator_leaking2 (test.test_exceptions.ExceptionTests) ... ok
test_generator_leaking3 (test.test_exceptions.ExceptionTests) ... ok
test_generator_leaking4 (test.test_exceptions.ExceptionTests) ... ok
test_generator_next_cleanup_exc_state (test.test_exceptions.ExceptionTests) ... ok
test_generator_send_cleanup_exc_state (test.test_exceptions.ExceptionTests) ... ok
test_generator_throw_cleanup_exc_state (test.test_exceptions.ExceptionTests) ... ok
test_memory_error_cleanup (test.test_exceptions.ExceptionTests) ... ok
test_recursion_error_cleanup (test.test_exceptions.ExceptionTests) ... ok
test_str (test.test_exceptions.ExceptionTests) ... ok
test_unhandled (test.test_exceptions.ExceptionTests) ... ok
test_unicode_change_attributes (test.test_exceptions.ExceptionTests) ... ok
test_unicode_errors_no_object (test.test_exceptions.ExceptionTests) ... ok
test_unraisable (test.test_exceptions.ExceptionTests) ... BEGINBEGINBEGINBEGINBEGIN
Exception ignored in: <bound method ExceptionTests.test_unraisable.<locals>.BrokenDel.__del__ of <test.test_exceptions.ExceptionTests.test_unraisable.<locals>.BrokenDel object at 0x7f4313a41ef8>>
Traceback (most recent call last):
  File "/home/irina/patrik/cpython/Lib/test/test_exceptions.py", line 1024, in __del__
    raise exc
ValueError: del is broken

ENDENDENDENDENDENDENDEND
BEGINBEGINBEGINBEGINBEGIN
Exception ignored in: <object repr() failed>
Traceback (most recent call last):
  File "/home/irina/patrik/cpython/Lib/test/test_exceptions.py", line 1024, in __del__
    raise exc
ValueError: del is broken

ENDENDENDENDENDENDENDEND
BEGINBEGINBEGINBEGINBEGIN
Exception ignored in: <bound method ExceptionTests.test_unraisable.<locals>.BrokenExceptionDel.__del__ of <test.test_exceptions.ExceptionTests.test_unraisable.<locals>.BrokenExceptionDel object at 0x7f4313a41ef8>>
Traceback (most recent call last):
  File "/home/irina/patrik/cpython/Lib/test/test_exceptions.py", line 1034, in __del__
    raise exc
test.test_exceptions.BrokenStrException: <exception str() failed>

ENDENDENDENDENDENDENDEND
ok
test_windows_message (test.test_exceptions.ExceptionTests)
Should fill in unknown error code in Windows error message ... skipped 'test specific to Windows'
test_attributes (test.test_exceptions.ImportErrorTests) ... ok
test_non_str_argument (test.test_exceptions.ImportErrorTests) ... ok
test_reset_attributes (test.test_exceptions.ImportErrorTests) ... ok

----------------------------------------------------------------------
Ran 44 tests in 0.158s

OK (skipped=1)
1 test OK.

Total duration: 363 ms
Tests result: SUCCESS




$ ./python ../coveragepy run --pylib --source=exceptions Lib/test/regrtest.py test_exceptions
Run tests sequentially
0:00:00 [1/1] test_exceptions
Exception ignored in: <bound method ExceptionTests.test_unraisable.<locals>.BrokenDel.__del__ of <test.test_exceptions.ExceptionTests.test_unraisable.<locals>.BrokenDel object at 0x7f1c67de69b0>>
Traceback (most recent call last):
  File "/home/irina/patrik/cpython/Lib/test/test_exceptions.py", line 1024, in __del__
    raise exc
ValueError: del is broken
BEGINBEGINBEGINBEGINBEGIN

ENDENDENDENDENDENDENDEND
Exception ignored in: <object repr() failed>
Traceback (most recent call last):
  File "/home/irina/patrik/cpython/Lib/test/test_exceptions.py", line 1024, in __del__
    raise exc
ValueError: del is broken
BEGINBEGINBEGINBEGINBEGIN

ENDENDENDENDENDENDENDEND
Exception ignored in: <bound method ExceptionTests.test_unraisable.<locals>.BrokenExceptionDel.__del__ of <test.test_exceptions.ExceptionTests.test_unraisable.<locals>.BrokenExceptionDel object at 0x7f1c67de6f60>>
Traceback (most recent call last):
  File "/home/irina/patrik/cpython/Lib/test/test_exceptions.py", line 1034, in __del__
    raise exc
test.test_exceptions.BrokenStrException: <exception str() failed>
BEGINBEGINBEGINBEGINBEGIN

ENDENDENDENDENDENDENDEND
test test_exceptions failed -- multiple errors occurred; run in verbose mode for details
test_exceptions failed

1 test failed:
    test_exceptions

Total duration: 1 sec
Tests result: FAILURE
Coverage.py warning: Module exceptions was never imported.
Coverage.py warning: No data was collected.



Somehow stuff is no longer ending up in stderr. That empty line is most likely the report that get printed when its value is "".