Port twisted.internet.main to Python 3

[itamar]

twisted.internet.main should run on Python 3.

[itamarst]

(In [35633]) Branching to 'imain-py3-5973'

[itamar]

Should work now.

Started, but haven't looked at:  http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/imain-py3-5973

[exarkun]

1. Add future imports to twisted/internet/main.py
2. Relying on addCleanup to put the original reactor back makes me nervous. What code is going to run between the test method returning and the addCleanup functions running? What if it tries to import twisted.internet.reactor? It seems safer to me to restore sys.modules['twisted.internet.reactor'] before the test method returns. (In fact, I wrote a utility for doing this in #5935 which I guess is basically ready to be merged.)
3. I don't understand the try/except or the new comment in test_alreadyInstalled.

[itamarst]

(In [35650]) Address review comments 1 and 3. Refs #5973

[itamarst]

(In [35651]) Branching to 'imain-py3-5973-2'

[itamarst]

(In [35653]) Address review comment 2. Refs #5973

[itamar]

OK, ready for review again. Started new buildbot run:  http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/imain-py3-5973-2

[exarkun]

1. Shouldn't NoReactor.__enter__ also delete twisted.internet.reactor if it exists? Also, I think self._currentReactor is synonymous with self.modules["twisted.internet.reactor"].

Looks good otherwise, please merge when you're happy with that one point.

[itamarst]

(In [35665]) Merge imain-py3-5973-2.

Author: itamar Review: exarkun Fixes: #5973

Port twisted.internet.main to Python 3.