- Changed the approach by which the "single inheritance criterion"

- Changed the approach by which the "single inheritance criterion"
is applied, when using :meth:`.Query.from_self`, or its common
user :meth:`.Query.count`.  The criteria to limit rows to those
with a certain type is now indicated on the inside subquery,
not the outside one, so that even if the "type" column is not
available in the columns clause, we can filter on it on the "inner"
query.
fixes #3177