Ensure iterable passed to Select is not a mapped class

Ensure iterable passed to Select is not a mapped class

Fixed regression caused by :ticket:`5395` where tuning back the check for
sequences in :func:`_sql.select` now caused failures when doing 2.0-style
querying with a mapped class that also happens to have an ``__iter__()``
method. Tuned the check some more to accommodate this as well as some other
interesting ``__iter__()`` scenarios.

Fixes: #6300
Change-Id: Idf1983fd764b91a7d5fa8117aee8a3def3cfe5ff