allow Executable to be accepted by Session.execute()

allow Executable to be accepted by Session.execute()

Fixed an issue where the API to create a custom executable SQL construct
using the ``sqlalchemy.ext.compiles`` extension according to the
documentation that's been up for many years would no longer function if
only ``Executable, ClauseElement`` were used as the base classes,
additional classes were needed if wanting to use
:meth:`_orm.Session.execute`. This has been resolved so that those extra
classes aren't needed.

Change-Id: I99b8acd88515c2a52842d62974199121e64c0381