Add Python 3 requirements to coding standard

[itamar]

All new code should be, at the very least, non-conflicting with Python 3. The coding standard should be updated appropriately. Some, but not all, items include:

• All byte strings must start with b.
• Exceptions must be caught except Exception as e.
• Use of new functions and classes in twisted.python.compat, e.g. unicode.
• All modules must have the division __future__.

Etc.

[exarkun]

(In [35889]) Branching to 'coding-standard-py3-5939'

[exarkun]

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

[itamar]

We can live with that for now until we have a complete list, yeah. The buildbot should catch most egregious problems, hopefully. Merge, with news file.

[exarkun]

(In [35913]) Merge coding-standard-py3-5939

Author: exarkun Reviewer: itamar Fixes: #5939

Add a note to the coding standard about paying attention to Python 3 compatibility.