add MariaDB placeholder impl
add MariaDB placeholder impl

in 599f27dcce62abac1f90a44f9c9a85e7dca885de we updated the
checks for "mariadb" to match SQLAlchemy 1.4.  Another
change if merged will add a new dialect name "mariadb" that's
basically the mysql dialect with a different name; forwards-support
that as well.

Change-Id: I6ba1c13304fa8d6f45dd6453f4c71d04021163ae

zzzeek/alembic,sqlalchemy/alembic