Xmpp-backends

* Add support for Python 3.7 and 3.8.
* Add support for Django 2.2 and 3.0.
* Add support for ejabberd 18.09.
* Drop support for Python 2.7 and 3.4.
* Drop support for Django 1.11, 2.0 and 2.1.
* Drop support for ejabberd 14.07.
* Run test suite with warnings turned into errors to make sure that deprecated Django code is found.
* Run test suite with coverage to check code coverage.
* Add more tests to increase test coverage

* Update requirements.
* Current xmpp-backends version is no available as a tuple in `xmpp_backends.VERSION`.
* Also test ejabberd versions 18.03, 18.04 and 18.06.
* Backend classes now get the API version directly from the backend via the `api_version` property.
* The `version` parameter and `get_version` functions are now deprecated in favor of `api_version`.
* Fully support Django 2.1.
* Modify the test-suite so that DeprecationWarnings from Django are an error.
* `all_user_sessions()` returns a valid status and status text for ejabberd >= 18.06 (the fields were added to the underlying API call).