Make `pip install -e` work

This allows `pip install -e` and `python setup.py develop` to work
without having to do the stuff in
http://docs.ansible.com/intro_installation.html#running-from-source so
it's a tad nicer for Python programmers who are accustomed to installing
packages as editable/in development mode.

Fixes GH-8355 (https://github.com/ansible/ansible/issues/8355)