Minimum supported versions are now Django 2.0, Python 3.5.
This touches a lot of code, to:
* Remove obsolete portability code and workarounds
(six, backports of email parsers, test utils, etc.)
* Use Python 3 syntax (class defs, raise ... from, etc.)
* Correct inheritance for mixin classes
* Fix outdated docs content and links
* Suppress Python 3 "unclosed SSLSocket" ResourceWarnings
that are beyond our control (in integration tests due to boto3,
python-sparkpost)
* Add Django 3.1 builds
* Django main is now 3.2dev; add test settings
* Rough in changelog deprecation notice for Django 1.11
and Python 2.7
* Travis-CI is apparently supporting pypy3 alias now
* Travis-CI has apparently added Python 3.8 release now
* Set up tox for testing supported Django/Python combinations
* Also include tox env for checking and building docs
* Use tox-travis for Travis CI integration
* Add tests against Django master
* Document building docs and running tests with tox
* Restructure runtests.py as suggested in
https://docs.djangoproject.com/en/1.9/topics/testing/advanced/#using-the-django-test-runner-to-test-reusable-applications
* Load version-specific Django settings modules
(rather than trying to make a single settings.configure()
work with all supported versions).
* Use `django-admin startproject` defaults for all
settings modules. (Tests compatibility with typical
apps, middleware, and other settings.)
* Set up tests-specific url config; switch to literal
urls in test cases. (Eliminates url `reverse` from
tests.)
* Make runtests.py executable
Closes#18
