hghave: move from requiring the PYTHON3 env var to looking for `python3`...
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
