Aiosql

Improve testing.

Features:
- add MariaDB support
- allow to change file extension when loading directories
- switch `mysql.connector` dependency to `mysql-connector-python` because `mysql-connector` is obsolete
- improved documentation

Tests:
- simplify github CI tests, no service needed
- run from Python 3.7 to 3.11
- extensive rework of tests to ignore missing modules, use marks…
- add tests with docker images, needed for mariadb
- depend on pytest 7.

- improved documentation
- add missing dependency on `setuptools`
- improved Makefile
- add untested `mariadb` support (will be tested and advertised in the next release)
- some refactoring (for next release really)

- improved CI checks with `rstcheck`
- dynamic query functions point to the SQL query file
- illegal query names such as `1st` are filtered out
- documentation has been improved
- some code refactoring…

Metainformation and documentation updates.

- Improved documentation and tests.
- normalize adapter names to lowercase.