Use the stdlib `os.initgroups` if it is available

[exarkun]

os.initgroups was added to the Python 2.7 standard library. We should prefer it over our twisted.python._initgroups extension.

[vperic]

As far as I can see, this is a simple matter of wrapping the current code with another "try/except" block. I've attached a patch doing just that. Not really sure how to test it, either.

[therve]

I think the tests are covering it already, and the builders on 2.7 will pick it up. But you should change twisted/topfiles/setup.py to not build _initgroups.c on 2.7 and newer. Please add a NEWS fragment as well (and merge). Thanks!

[exarkun]

But you should change twisted/topfiles/setup.py to not build _initgroups.c on 2.7 and newer

With unit tests, of course.

[exarkun]

(In [35884]) Branching to 'stdlib-initgroups-4920'

[exarkun]

Done and tested:  http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/stdlib-initgroups-4920

[itamar]

There's a failure on one of the builds:  http://buildbot.twistedmatrix.com/builders/debian-easy-py2.6-epoll/builds/401/steps/epoll/logs/problems

Other than that, looks good; merge when fixed.

[exarkun]

(In [35900]) Merge stdlib-initgroups-4920

Author: exarkun Reviewer: itamar Fixes: #4920

Use os.initgroups when it is available (and skip building our extension module for the same functionality in that case).