mercurial: use pure Python module policy on Python 3
The C extensions don't yet work with Python 3. Let's minimize the
work required to get Mercurial running on Python 3 by always using
the pure Python module policy on Python 3.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state