Switch from pymysql to mariadb and a connection pool

[RuedigerVoigt]

Currently exoskeleton uses pymysql to connect with MariaDB. Recently the MariaDB Connector/Python was published. See:

• https://mariadb.com/resources/blog/how-to-connect-python-programs-to-mariadb/
• https://pypi.org/project/mariadb/

It would bring real connection pooling.

However, it relies on the C-Connector. A test with a system based on Ubuntu 18 showed that the installed version of this is not compatible with the new Python connector. So a user would have to upgrade the mariadb-server to a version higher than supplied by the OS, and / or integrate some repositories. Both can be quite a hurdle

As pymysql is "good enough" the switch is postponed to exoskeleton 2.