Version support changes (possibly breaking)
* Drop support for Python 2.7
* Drop support for Python 3.4
* Drop support for Django 1.8
* Drop support for Django 1.10
* Add support for Django 2.1
* Add support for Django 2.2
* Add support for Django 3.0
* Add support for Python 3.7
* Add support for Python 3.8
Additions and bugfixes
* Bug: Fix EnumSupportSerializerMixin for non-editable fields
* Docs: Readme improvements
* Feature: Warn when some values of an enum won't fit in the backing database field
* Packaging: PEP-396 compliant `__version__` attribute
* Packaging: Tests are now packaged with the source distribution