This adds a lot of new out-of-the-box functionality for running various

test matrices. There's now a pre-defined list of databases, covering

SQLite, MySQL 5.6/5.7/8, MariaDB, and Postgress 11.8/12/13/14. These are

configured in default_test_db_settings.py, which is automatically

loaded, and a tests/docker-compose.yaml is provided in order to fetch

and spin up each of the databases.
The tox configuration now contains a number of useful environments for

performing tests against various Python/Django/database combinations.

Along with the default Python/Django environments that are run, it

introduces the following new environments:


djangos: Tests all versions of Django against a pre-determined

  version of Python.


pythons: Tests all versions of Python against a pre-determined

  version of Django.


dbtests-<dbname>: Tests all versions of Django with the provided

  database against a pre-determined version of Python.


dbtests-<dbname>-<mysql_backend>: Similar to dbtests-<dbname>,

  but accepts a -myisam or -innodb suffix to test using that engine

  (only supported for MySQL or MariaDB)


dbtests-newest: Tests all versions of Django with the newest

  testable versions of each type of database.


dbtests-minmax: Tests all versions of Django with the oldest and

  newest testable versions of each type of database.


dbtests-all: Tests all versions of Django with all databases.


During test run, pytest now provides more relevant database

information. Previously, it would provide the MySQL, Postgres, or SQLite

backend versions, regardless of which tests were being run. Now it

provides them only if testing that backend.
For MySQL/MariaDB, we also show the database storage backend that's

being tested with.
Skipped tests are now shown in pytest output.
This change really does strain tox's configuration options a bit.

Unfortunately, there isn't a way to make generative sections that easily

inherit from a previous one without redundant inheritance rules, and

there isn't a way to list optional test factors (like

myisam/innodb), nested factors (like -{mysql{5_6,5_7},postgres{11,12}},

or pre-configured but non-default test environments. Some of this may

improve down the road with tox 4.

Tested each of the new environments, letting some run overnight.
Been using most of these for testing with recent changes.