Use label reference coercion for select() distinct keyword argument

Use label reference coercion for select() distinct keyword argument

Fixed bug where "distinct" keyword passed to :func:`.select` would not
treat a string value as a "label reference" in the same way that the
:meth:`.select.distinct` does; it would instead raise unconditionally. This
keyword argument and the others passed to :func:`.select` will ultimately
be deprecated for SQLAlchemy 2.0.

Fixes: #5028
Change-Id: Id36cfe477ed836c3248824ce1b81d0016dbe99f4