I think the time has come. My main reason for wanting to do this is
to force Mercurial developers to use Python 3 in their day-to-day work.
This should help flush out any remaining Python 3 bugs.
If this change is too controversial, we can revert it before the next
Makefile now uses `python3` instead of `python` by default. 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`.