re-support mysql-connector python

re-support mysql-connector python

Support has been re-added for the MySQL-Connector/Python DBAPI using the
``mysql+mysqlconnector://`` URL scheme.   The DBAPI now works against
modern MySQL versions as well as MariaDB versions (in the latter case it's
required to pass charset/collation explicitly).   Note however that
server side cursor support is disabled due to unresolved issues with this
driver.

References: #12332
Change-Id: I81279478196e830d3c0d5f24ecb3fe2dc18d4ca6