ie make sure both that it's a valid allocation, but also that
it will have Python destructor semantics applied when it is freed
(as opposed to, say, STLCompatAllocator-allocated memory).

This is to make sure that extension modules don't use a different
allocation routine than we expected.

There are only a few specialized places that I could find that we
actually want the old-behavior; in dump(), and in PyObject_Init right
before we call setIsPythonObject.  So for those cases, add a new
isValidGCMemory call that doesn't do the allocation-kind check.