assign variant mapping on adapt()

assign variant mapping on adapt()

Fixed regression from the 1.4 series where the refactor of the
:meth:`_types.TypeEngine.with_variant` method introduced at
:ref:`change_6980` failed to accommodate for the ``.copy()`` method, which
will lose the variant mappings that are set up. This becomes an issue for
the very specific case of a "schema" type, which includes types such as
:class:`.Enum` and :class:`.ARRAY`, when they are then used in the context
of an ORM Declarative mapping with mixins where copying of types comes into
play.  The variant mapping is now copied as well.

Fixes: #11176
Change-Id: Icf1a2752f60fce863c87ead8b0fe298b0f3d3766
(cherry picked from commit 29a428955a904e235e1b85e928cbe89155aeca82)