Deprecate plain string passed to session.query()

Deprecate plain string passed to session.query()

Passing a plain string expression to :meth:`.Session.query` is deprecated,
as all string coercions were removed in :ticket:`4481` and this one should
have been included.   The :func:`.literal_column` function may be used to
produce a textual column expression.

Note this changeset is in the 1.3 branch only.  The 1.4 (currently master)
branch will already have removed this behavior.

Fixes: #4873
Change-Id: I20067a6707c7e2f028c67b1fbf0e8e4d9d136233