hghave: move from requiring the PYTHON3 env var to looking for `python3` We're starting to write new utility scripts in Python 3, and I think it's reasonable to expect `python3` to be something we should use if it's present. I wrote the regex to specifically ban Python 3 prior to Python 3.5 because we won't ever support that. I moved the only test that was using $PYTHON3 to just using `python3` directly. Differential Revision: https://phab.mercurial-scm.org/D4290