tests: add support in run-tests.py for an environment variable to stop color I don't like the color output, so this gives me a way to easily never see it while still having pygments on my machine.

setup: fix mistake that prevented Python 3 from being excluded My intent was to only allow Python 3 if the right environment variable is set (for when people want to use `pip install .` on hg locally). This fixes the bug in my previous change. I verified that `python3.6 run-tests.py` still passes the tests that passed before, and that all tests pass on 2.7 (including our virtualenv-using installation test). Differential Revision: https://phab.mercurial-scm.org/D185