It turns out the warnings.catch_warnings version has re-entrancy issues
due to the fact that it can't use @contextmanager:

Python 2.6b3+ (trunk:66143M, Sep  2 2008, 20:04:43)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import warnings
>>> orig_filters = warnings.filters
>>> cw = warnings.catch_warnings()
>>> with cw:
...   warnings.filters = []
...   with cw:
...     pass
...
>>> warnings.filters is orig_filters
False
>>> warnings.filters
[]
>>> orig_filters
[('ignore', None, <type 'exceptions.PendingDeprecationWarning'>, None,
0), ('ignore', None, <type 'exceptions.ImportWarning'>, None, 0),
('ignore', None, <type 'exceptions.BytesWarning'>, None, 0)]

I propose that we just revert to the test.test_support.catch_warnings
implementation that was used in the beta releases, and leave the
question of whether to expose this ability somewhere other than our own
regression test support module for 2.7/3.1. That version worked, and the
attempt to move it at the last minute has caused nothing but trouble.

So on trunk we would revert the following checkins:
r66135 (relocate to warnings and change API)
r66321 (change API again in attempt to fix bugs in r66135)

And on the py3k branch we would revert:
r66139 (merge r66135)
r66322* (merge r66322)

*This commit actually appears to have missed the changes to
test.test_support that were in r66321 - test.support was not modified by
the r66322 checkin (which strikes me as all the more reason to revert
all of these changes and go back to the beta implementation)