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.
#require test-repo
$ cd "$TESTDIR"/..
New errors are not allowed. Warnings are strongly discouraged.
$ hg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
> python contrib/check-config.py