I don't know if these are related to the latest changes, but they only appear on the 3.3 buildbots:

======================================================================
FAIL: test_case_insensitivity (importlib.test.extension.test_case_sensitivity.ExtensionModuleCaseSensitivityTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\cygwin\home\db3l\buildarea\3.x.bolen-windows7\build\lib\importlib\test\extension\test_case_sensitivity.py", line 30, in test_case_insensitivity
    self.assertTrue(hasattr(loader, 'load_module'))
AssertionError: False is not true

======================================================================
FAIL: test_insensitive (importlib.test.source.test_case_sensitivity.CaseSensitivityTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\cygwin\home\db3l\buildarea\3.x.bolen-windows7\build\lib\importlib\test\source\test_case_sensitivity.py", line 51, in test_insensitive
    self.assertTrue(hasattr(insensitive, 'load_module'))
AssertionError: False is not true

I was getting that error the other day on my OS X laptop, but then I thought I tweaked the code well enough to solve it (since I am running on a case-insensitive filesystem as well). It seems to stem from what nt.environ contains when the test sets PYTHONCASEOK and importlib._bootstrap._case_ok() tries to see if b'PYTHONCASEOK' is in nt.environ.

You (or anyone) have a Windows box to quickly test what nt.environ contains after os.environ.set('PYTHONCASEOK', '1') is called?

Otherwise it's the nt.listdir() comparison that's failing. Probably should check that is returning reasonable stuff as well.

Well apparently nt.environ doesn't reflect os.environ:

>>> os.environ['PYTHONCASEOK'] = '1'
>>> nt.environ['PYTHONCASEOK']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'PYTHONCASEOK'
>>> nt.environ[b'PYTHONCASEOK']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: b'PYTHONCASEOK'
>>> os.environ['PYTHONCASEOK']
'1'
>>> os.environ[b'PYTHONCASEOK']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\t\cpython\lib\os.py", line 450, in __getitem__
    value = self._data[self.encodekey(key)]
  File "C:\t\cpython\lib\os.py", line 508, in encodekey
    return encode(key).upper()
  File "C:\t\cpython\lib\os.py", line 503, in check_str
    raise TypeError("str expected, not %s" % type(value).__name__)
TypeError: str expected, not bytes

This is silly and is because of how the environ mapping is implemented in Lib/os.py: under POXIX, os.environ reflects posix.environ (it uses the same underlying dict), while under Windows, os.environ uses a distinct dict from nt.environ.

Is there a technological reason environ is not updated, or is it simply
oversight?

Lib/os.py: under POXIX, os.environ reflects posix.environ (it uses the same
underlying dict), while under Windows, os.environ uses a distinct dict from
nt.environ.
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue13890>
> _______________________________________
>

I think it's more laziness. _Environ.__setitem__ could also update the original mapping.

New changeset 54d7823ec488 by Brett Cannon in branch 'default':
Issue #13890: Fix importlib case-sensitivity tests to not run on Windows.
http://hg.python.org/cpython/rev/54d7823ec488

New changeset 2914ce82bf89 by Brett Cannon in branch 'default':
Issue #13890: Also fix for extension module tests for case-insensitivity.
http://hg.python.org/cpython/rev/2914ce82bf89