Depend on PyMySQL (rather than MySQLdb)

[jwarnier]

Please switch to depending on PyMySQL by default instead of mysqlclient (MySQLdb).
This would, by default, limit dependencies and ease installation in most cases and most target environments (e.g. exotic CPU architectures).
As an example, this would allow significantly simplifying and shrinking Docker images.

See https://github.com/pymysql/pymysql.