This enables support for Django 3.0. There were only two major

difference between Django 2.2 and 3.0 that impacts Django Evolution:

The removal of django.utils.six.
A change to the internal structure of applied migration lists in the

   migration machinery, which we override in order to inject our own

   state.

Those were both primarily taken care of by the past two commits. This

commit builds upon the new MigrationList class to transform to the

proper internal structure, depending on the version of Django.
There's also a small change to the ordering of SQL statements generated

when constructing a ManyToManyField's intermediary table on MySQL and

Postgres, which just requires some shuffling in the SQL test mapping

files.
Other than that, there isn't really anything left that's specific to

Django 3.0.
The documentation and package dependencies have been updated to include

Django 3.0 support.

Unit tests pass for all supported versions of Python and Django.