Tox: Support 2to3 configurations / test file postprocessing

[obestwalter]

from ISSUES.txt originally created by @hpk42

Tests should be copied and transformed before being executed.
Setup a test definition::

[testenv]
testsrc=testing/**.py 
commands=py.test {posargs}

[testenv:py3*]
use2to3 = True

would transform all specified test files for all py3 environments
such that a subsequent test run::

$ tox -e py25,py32 testing/test_hello.py

causes a transformation of the test files to prepare test runs on
Python3 interpreters. The posargs specified files will be rewritten
to point to the transformed test files.

[The-Compiler]

Not sure if this is still relevant - almost everyone seems to have moved to using six and having polyglot python 2/3 code nowadays.

[obestwalter]

True. Would also vote for closing as wontfix.

[nicoddemus]

Same here, I vote to close as wontfix.