Make setup.py on Python 3 create public API versions of _XXXpy3.py modules

[itamar]

Currently for Python 3 we have a number of modules that are only partially ported, with names like _foopy3.py for the ported parts of foo.py. Only the former is installed on Python 3 (see #6040). We could have users of Twisted on Python 3 use these private modules in their code temporarily, but that means it's harder for them to support both Python 2 and Python 3, and they'll have to change their code once these temporary modules go away.

Another alternative is to have the installed version of Twisted on Python 3 fix this issue. In particular, for twisted/bar/_foopy3.py, setup.py should install a corresponding twisted/bar/foo.py with the following code:

from twisted.bar._foopy3 import *
__all__ = list(locals().keys())

Or something to that effect.

Given we're not guaranteeing backwards compat for now on Python 3, this is more of a nice-to-have thing.

[itamar]

All _foopy3 modules will soon be gone.