Django-simplesshkey

* Add support for Django 4.0, 4.1, 4.2
* Add CI with linter and tests
* Remove unused setting `SSHKEY_AUTHORIZED_KEYS_OPTIONS` (leftover from fork)

* Add support for Django 3.2
* Drop support for Django < 3.0

* Add compatibility with Django 2.0

* Ensure consistent unicode behaviour between python2 and python3.
* Minor admin improvement.

* Don't crash when parsing malformed SSH key.

* Remove integration with OpenSSH server: key lookup, scripts, etc.
* Stop sending emails when SSH keys are added or modified. It's much more flexible to do this directly in the application that uses django-simplesshkey, using signals.
* Impose less constraints on the SSH keys: allow duplicate keys, keys without name...
* Allow to subclass the UserKey model: can be useful to override some fields.