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 docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: