makefile: use Python 3 by default (BC)
This change is long overdue IMO.
.. bc::
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
# invalid filenames
test-add.t
test-init.t
test-clone.t
test-contrib.t
test-hgweb-raw.t
test-walk.t
# no sockets or fifos
test-hup.t
# no hardlinks
test-hardlinks.t
test-relink.t
# exec bit problems
test-convert-bzr-114.t
test-convert-bzr-directories.t
test-convert-bzr-merges.t
test-convert-bzr-treeroot.t
test-convert-darcs.t
test-merge-tools.t
# debugstate exec bit false positives
test-dirstate.t
test-filebranch.t
test-merge-remove.t