have automap suppress overlaps warning for mapped secondary

have automap suppress overlaps warning for mapped secondary

Fixed regression in :mod:`sqlalchemy.ext.automap` extension such that the
use case of creating an explicit mapped class to a table that is also the
:paramref:`_orm.relationship.secondary` element of a
:func:`_orm.relationship` that automap will be generating would emit the
"overlaps" warnings introduced in 1.4 and discussed at :ref:`error_qzyx`.
While generating this case from automap is still subject to the same
caveats that the "overlaps" warning refers towards, as automap is intended
for more ad-hoc use cases, the condition which produces the warning is
disabled when a many-to-many relationship with this particular pattern is
generated.

Fixes: #6679
Change-Id: Ib3a53982b076ed4999b0d3235f84008b9e2f1cce