Fix a bug caused by the garbage collector clearing weakrefs too early. The
weakrefs in the ``tp_subclasses`` dictionary are needed in order to correctly
invalidate type caches (for example, by calling ``PyType_Modified()``).
Clearing weakrefs before calling finalizers causes the caches to not be
correctly invalidated. That can cause crashes since the caches can refer to
invalid objects. Defer the clearing of weakrefs without callbacks until after
finalizers are executed.
I have compared output between pre- and post-patch runs of these tests
to make sure there's nothing missing and nothing broken, on both
Windows and Linux. The only differences I found were actually tests
that were previously *not* run.
