Use importlib_metadata; add namespace for mariadb

The ``importlib_metadata`` library is used to scan for setuptools
entrypoints rather than pkg_resources.   as importlib_metadata is a small
library that is included as of Python 3.8, the compatibility library is
installed as a dependency for Python versions older than 3.8.

Unfortunately setuptools "attr:" is broken because it tries to import
the module; seems like this is fixed as part of
https://github.com/pypa/setuptools/pull/1753 however this is too recent
to rely upon for now.

Added a new dialect token "mariadb" that may be used in place of "mysql" in
the :func:`_sa.create_engine` URL.  This will deliver a MariaDB dialect
subclass of the MySQLDialect in use that forces the "is_mariadb" flag to
True.  The dialect will raise an error if a server version string that does
not indicate MariaDB in use is received.   This is useful for
MariaDB-specific testing scenarios as well as to support applications that
are hardcoding to MariaDB-only concepts.  As MariaDB and MySQL featuresets
and usage patterns continue to diverge, this pattern may become more
prominent.

Fixes: #5400
Fixes: #5496
Change-Id: I330815ebe572b6a9818377da56621397335fa702

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