For some yet unknown reason, test_codecs fails when it is run after
test_zipimport and subsequently test_mailbox:

./python Lib/test/regrtest.py -w test_zipimport test_mailbox test_codecs
[SNIP]
======================================================================
ERROR: test_basics (test.test_codecs.BasicUnicodeTest)
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Traceback (most recent call last):
  File "/home/alex/src/python.org/py3k/Lib/test/test_codecs.py", line
1360, in test_basics
    encodedresult += encoder.encode(c)
  File "/home/alex/src/python.org/py3k/Lib/encodings/ascii.py", line 22,
in encode
    return codecs.ascii_encode(input, self.errors)[0]
AttributeError: 'NoneType' object has no attribute 'ascii_encode'

======================================================================
ERROR: test_decoder_state (test.test_codecs.BasicUnicodeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/alex/src/python.org/py3k/Lib/test/test_codecs.py", line
1445, in test_decoder_state
    self.check_state_handling_decode(encoding, u, u.encode(encoding))
  File "/home/alex/src/python.org/py3k/Lib/test/test_codecs.py", line
30, in check_state_handling_decode
    part1 = d.decode(s[:i])
  File "/home/alex/src/python.org/py3k/Lib/encodings/ascii.py", line 26,
in decode
    return codecs.ascii_decode(input, self.errors)[0]
AttributeError: 'NoneType' object has no attribute 'ascii_decode'

----------------------------------------------------------------------
Ran 121 tests in 0.204s

FAILED (errors=2)
test test_codecs failed -- errors occurred; run in verbose mode for details