Port qual, safe_repr and safe_str (from twisted.python.reflect) to Python 3

[itamar]

qual, safe_repr and safe_str are used by twisted.python.failure and twisted.python.log, so it would be useful to port them to Python 3.

[itamarst]

(In [35452]) Branching to 'reflect-python3-5913'

[itamar]

OK, ready for review. I started build:

 http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/reflect-python3-5913

[exarkun]

1. Argh StringIO/BytesIO. If you think it's a good idea, perhaps twisted.python.compat.NativeStringIO? Or we could wait until we need this check again, I guess, and refactor then.
2. Explicit documentation about the return type of safe_str and safe_repr would probably be good at this point.
3. The docstring for unsignedID says it is mostly only necessary before Python 2.5. I guess keeping the existing implementation doesn't hurt, though.

Otherwise good. Fix at least point 2 and then merge, thanks.

[itamar]

1. Opened #5930.
2. Done.
3. Yeah, I noticed that too late. Plus probably Python 3.5 will make negative ids again, so can't hurt.

[itamarst]

(In [35463]) Branching to 'reflect-python3-5913-2'

[itamarst]

(In [35466]) Merge reflect-python3-5913-2

Author: itamar Review: exarkun Fixes: #5913

More functions from twisted.python.reflect ported to Python 3.