Restore (and test more reliabley) twisted.python.log's capture of warnings

[Itamar Turner-Trauring]

Warnings are supposed to end up in the Twisted log once logging is initialized, but #5885 broke that functionality. It should be fixed (reverting #5885 would be somewhat painful at this point) and the tests improved so this doesn't happen again.

[itamarst]

(In [36022]) Branching to 'warnings-6087'

[Itamar Turner-Trauring]

I've fixed it, I think. Note that this involves a minor change in the way we do things; when we write a warning to a file in LogPublisher.showwarning, we use the *original* warnings.showwarning (or at least, what was there at import time). Previously we used the version captured by startLoggingWithObserver, which caused problems using LogPublisher.showwarning before logging had been initialized. It is vaguely possible this will break some obscure code somewhere that overrides showwarning sufficiently to actually write to files differently, and does the override in between importing twisted.python.log and starting twisted logging. I am inclined not to care, but I could come up with a different solution.

​http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/warnings-6087

[Jean-Paul Calderone]

Thanks.

1. in test_publisherWarningInitialization
   1. the builtin setattr is preferable to using warnings.__setattr__.
2. twisted/python/log.py
   1. As discussed on IRC, the showwarning APIs here aren't intended to be used by applications. I think we should move towards deprecating them and not worry about all the super obscure edge cases in their behavior (related to what order events happen in, etc).

Not sure many more review comments make sense, given that hopefully the last point above means things will change significantly and for the better.

[Itamar Turner-Trauring]

​http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/warnings-6087 has started.

I implemented a simpler "fix"; we're back to the previous behavior from before the Python 3 changes, I believe, with some tests skipped or munged until they work.

[Jean-Paul Calderone]

Okay. Please merge.

[itamarst]

(In [36349]) Merge warnings-6087.

Author: itamar Review: exarkun Fixes: #6087

The Python 3 port of logging mistakenly broke warning integration; this fixes it. It's a .misc because from point of view of upgrading user nothing will have changed.