ignore and warn for native_enum=False with pg.ENUM datatype

ignore and warn for native_enum=False with pg.ENUM datatype

any UPPERCASE datatype refers to that exact type name rendered
on the database.   So PG's ENUM must render "ENUM" and is
"native" by definition.  warn if this flag is passed.

The :class:`_postgresql.ENUM` datatype is PostgreSQL-native and therefore
should not be used with the ``native_enum=False`` flag. This flag is now
ignored if passed to the :class:`_postgresql.ENUM` datatype and a warning
is emitted; previously the flag would cause the type object to fail to
function correctly.

Fixes: #6106
Change-Id: I08e0ec6fcfafd068e1eaf6aec13c8010f09ce94a