replace Variant with direct feature inside of TypeEngine

replace Variant with direct feature inside of TypeEngine

The :meth:`_sqltypes.TypeEngine.with_variant` method now returns a copy of
the original :class:`_sqltypes.TypeEngine` object, rather than wrapping it
inside the ``Variant`` class, which is effectively removed (the import
symbol remains for backwards compatibility with code that may be testing
for this symbol). While the previous approach maintained in-Python
behaviors, maintaining the original type allows for clearer type checking
and debugging.

Fixes: #6980
Change-Id: I158c7e56306b886b5b82b040205c428a5c4a242c