Django-migration-linter

* Stop packaging the 'tests' module into the release wheel file
* Add Django 3.2 support

* Remove `toml` support for config files

* Remove `.editorconfig` from default config file
* Allow utf-8 encoding in config files

**Renamed lint checks**:
* `REVERSIBLE_DATA_MIGRATION` -> `RUNPYTHON_REVERSIBLE`
* `NAMING_CONVENTION_RUNPYTHON_ARGS` -> `RUNPYTHON_ARGS_NAMING_CONVENTION`
* `DATA_MIGRATION_MODEL_IMPORT` -> `RUNPYTHON_MODEL_IMPORT`
* `DATA_MIGRATION_MODEL_VARIABLE_NAME` -> `RUNPYTHON_MODEL_VARIABLE_NAME`
* `REVERSIBLE_RUNSQL_DATA_MIGRATION` -> `RUNSQL_REVERSIBLE`

Features/fixes:
* Add Python 3.9 support
* Make data migration model import error less strict (issue 121)
* Add warning detection on RunPython call when model variable name is not the same as model class name
* Run checks on RunSQL migration operations
* Rename the RunPython data migration lint checks for improved consistency
* Refactor and commonise the loggers, so that all modules the `django_migration_linter` logger
* Allow using configuration file for linter options
* Add option `--include-name` and `--include-name-contains` to only include migration with a certain name

* Add possibility to lint newly generated migrations through the `makemigrations` command.
You can activate it through the `--lint` command option, or by default with the `MIGRATION_LINTER_OVERRIDE_MAKEMIGRATIONS = True` Django settings.

* Handle making a column NOT NULL with a Django default value as backward incompatible.
This should have been the case from the beginning, but it was not.
When one uses the `django-add-default-value` lib, the operation should still be compatible.
* Improve behaviour of the `--include-migrations-from` option.
When the given filename is missplelled, fail instead of linting all migrations.
When the file is empty, lint no migrations instead of linting all migrations.
* Update libraries used for testing (`tox`, `flake8`, etc.)