makefile: use Python 3 by default (BC)
This change is long overdue IMO.
Makefile now uses `python3` instead of `python` by default on non-Windows platforms. This means Mercurial will be built and run with Python 3 instead of Python 2.7 by default. To continue using Python 2, set the PYTHON variable. e.g. `make install PYTHON=python2.7`.
Differential Revision: https://phab.mercurial-scm.org/D7258