policy: add cffi policy for PyPy
This adds cffi policy in the case where we don't want to use C modules,
but instead we're happy to rely on cffi (bundled with pypy)
#require test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ 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