Merges from a release branch to any other branch are forbidden, this
hook has been added as a reminder to maintainers not to attempt it.

Pylint's --fail-under option is identical; so… why not?

Adds new options --editable/--non-editable for users to enable/disable
editable installs which are only supported by projects using
setuptools.

The option defaults to --editable if setup.py is found,
else --non-editable.

This is part of future work, if implemented at all.

* Add '--upgrade' flag
* Comma separate extras names