Add already ported Python 3 modules to porting howto

[itamar]

We should document API differences/pecularities in already ported modules in the Python 3 porting howto.

[exarkun]

I sort of punted, maybe it's good enough. Build results:  http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/coding-standard-py3-5939

[exarkun]

Sorry. Wrong ticket.

[exarkun]

Hmm. I wish this ticket had a list of what modules were in that "already ported" list. :/

[exarkun]

I just looked at what we've ported already and tried to catch changes. Found a few. We should try to be careful going forward to keep this porting guide up to date, rather than forgetting and then have to go back and figure out what changed.

[exarkun]

(In [35937]) Branching to 'more-python3-porting-guide-5956'

[itamar]

Arguably the fact that FilePath stayed as bytes is worth nothing, even if in theory nothing changed (consider all the unit tests we needed to update), so might be worth mentioning. And yeah, we should make sure changes end up here. Add mention of FilePath, then merge.

[exarkun]

(In [35953]) Note FilePath

refs #5956

[exarkun]

(In [35954]) Merge more-python3-porting-guide-5956

Author: exarkun Reviewer: itamar Fixes: #5956

Add some notes about potential traps to application developers who are porting a Twisted-based application to Python 3.