2.6.6 has an erronenous use of a unittest method that was added in 2.7, but it's only seen when running as root.  More specificially, with this guard:
    if posix.getuid() == 0 and hasattr(posix, 'getgroups') and sys.platform != 'darwin':

[david@surprise 2.6-clean]$ sudo ./python -m test.test_posix
   (snip)
======================================================================
ERROR: test_setgroups (__main__.PosixGroupsTester)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib64/python2.6/test/test_posix.py", line 356, in test_setgroups
    self.assertListEqual(groups, posix.getgroups())
AttributeError: 'PosixGroupsTester' object has no attribute 'assertListEqual'

----------------------------------------------------------------------
Ran 29 tests in 0.020s

FAILED (errors=1)

Seems to have come from r83126.  Fix would seem to be to replace the assertListEqual with AssertEqual.  Am attaching a patch.

I’m afraid tests are no exception to the rule “only security fixes in stable branches”.

TBH, the fact that this test only runs under sudo is more troublesome.  It's clearly a test that doesn't get run much.  E.g. it sneaked through pre-release testing and doesn't affect the buildbots.

But I agree that this shouldn't be applied to the 2.6 tree.