Django-twilio

upgrades django dynamic fixture so the test suite (on django 3.1) passes again. removal of south docs/usage. correction of twilio python docs URL.

fixes issues:
177
178

180 resolved via 181 183 , but unfortunately tests are breaking due to a downstream package which will need a code change. Otherwise, this should work.

django 2.2 and 3.0 only. python 3.5 - 3.8

there is some code change 169 and documentation change 172 but otherwise this is business as usual in terms of firming up our compatibility story with django.

once testing/community feedback turns positive the next will likely be an rc1 (just to have an rc), and then assuming no additional code change, straight to `0.13.0`.

Whereas 0.12.0 had an installation issue, 0.12.1 is the _same_ code except setup.py has been fixed.

For django 3.0 support, this is the base version.

This is on PyPI (currently), though it may be removed. It has an issue with the setup.py such that via setup.py django 3.0 cannot be installed.

0.12.1 is where you'll find the django 3.0 support via normal installation methods.

If you're on django 2.2 for instance, this will work fine, but again, not really the point of the upgrade :)

This implements 162 so we intend to support Django 3.0.