Fix repr for MySQL SET, generic Enum

Fix repr for MySQL SET, generic Enum

Fixed issues in :class:`_mysql.SET` datatype as well as :class:`.Enum`
where the ``__repr__()`` method would not render all optional parameters in
the string output, impacting the use of these types in Alembic
autogenerate. Pull request for MySQL courtesy Yuki Nishimine.

Fixes: #7720
Fixes: #7789
Closes: #7772
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7772
Pull-request-sha: d58845479f497f6b2e12d7df2e9eb2d6ac22109b
Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: Idcec23eab4258511d9f32f4e3d78e511ea6021f1
(cherry picked from commit a926dea6b78c91b627f0f0b86cdc6a9279872e99)