- Repaired support for Postgresql UUID types in conjunction with

- Repaired support for Postgresql UUID types in conjunction with
the ARRAY type when using psycopg2.  The psycopg2 dialect now
employs use of the psycopg2.extras.register_uuid() hook
so that UUID values are always passed to/from the DBAPI as
UUID() objects.   The :paramref:`.UUID.as_uuid` flag is still
honored, except with psycopg2 we need to convert returned
UUID objects back into strings when this is disabled.
fixes #2940

(cherry picked from commit b2189da65019ed2f44e77933a122619489319c5a)