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* Fix outdated docs content and links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)
Tox:
* Add Tox factor for extras (all, none, individual ESP).
For now, only break out ESPs that have specific extra
dependencies (amazon_ses, sparkpost).
* Install most package dependencies (including extras)
through the package itself.
* Use new runtests.py environment vars to limit test tags
when Tox isn't installing all extras.
Travis:
* Rework matrix to request specific TOXENVs directly;
drop tox-travis.
Test runner (runtests.py):
* Centralize RUN_LIVE_TESTS logic in runtests.py
* Add ANYMAIL_ONLY_TEST and ANYMAIL_SKIP_TESTS env vars
(comma-separated lists of tags)
Test implementations:
* Tag all ESP-specific tests with ESP
* Tag live tests with "live"
* Don't import ESP-specific packages at test module level.
(Test discovery imports test modules before tag-based filtering.)
Closes#104
