mox3 fails for context manager based mock testing

[Davanum Srinivas (DIMS)]

Apparently, we (OpenStack) own mox3 :) as our CI pushed the mox3 release into pypi.

It seems like the code is from Monty's repo:
https://github.com/emonty/pymox

There is no entries for mox3 in git.openstack.org or on launchpad....So i am logging the bug here for now.

Please see attached test case, based on code from here:
http://stackoverflow.com/questions/8905472/python-unit-test-how-to-use-mox-to-mock-the-gzip-with-statement

I stumbled into this one when updating Nova to use mox3:
https://review.openstack.org/#/c/135944/

======================================================================
FAIL: mox3.tests.test_gzip.MockContextManagerTest.test_generate_gzip_file
tags: worker-0
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/dims/openstack/pymox/mox3/tests/test_gzip.py", line 43, in test_generate_gzip_file
    self._generate_gzip_file()
  File "/Users/dims/openstack/pymox/mox3/tests/test_gzip.py", line 30, in _generate_gzip_file
    with gzip.GzipFile('file_name.txt.gz', 'wb') as f:
AttributeError: __exit__
Ran 1 (-185) tests in 0.001s (-0.100s)
FAILED (id=1, failures=1 (-29))
error: testr failed (1)