* Omit default_app_config under Django 3.2 and later
to avoid DeprecationWarning
* Also adds testing on Python 3.9, 3.10-alpha
* Also updates test matrix to include all supported
Python versions (some older Django versions expanded
Python support in patch releases)
Related changes:
* remove Travis-CI config; stop running tests on Travis
* rename live integration test environment variables
to all start with `ANYMAIL_TEST_` (simplifies tox config)