From what I can tell only place where file mode is tested is test_mode() method of class TestFilemode in test_stat.py. Relevant section for Windows (os.name != 'posix'):

with open(TESTFN, 'w'):
  pass

...

os.chmod(TESTFN, 0o700)
st_mode, modestr = self.get_mode()
self.assertEqual(modestr[:3], '-rw')
self.assertS_IS("REG", st_mode)
self.assertEqual(self.statmod.S_IFMT(st_mode),
                 self.statmod.S_IFREG)

Doesn't test that files under Windows can have only two modes:
- 0o444 - If os.chmod() is missing stat.S_IWRITE. Effectively readonly.
- 0o666 - Regular file by default.

On the top of that I believe last two assertions are equivalent - test file is regular. But that would be for a different issue, I guess.