tests/test-check-format.t

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

#require black test-repo

  $ cd $RUNTESTDIR/..
  $ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`