type_traverse() called for non-heap type

[John A Meinel]

I was trying to add a smoke test for https://code.launchpad.net/~jelmer/meliae/itervalues/+merge/385313

However, in Python 3+ I get failures of the form:
Fatal Python error: type_traverse() called for non-heap type '_ctypes.PyCFuncPtr'
Fatal Python error: type_traverse() called for non-heap type '_ctypes.Union'

I don't know if there is an obvious way to determine if a type is a 'non-heap type' and so we can avoid calling type_traverse for these objects.

It doesn't seem like writing the test to work around it is appropriate, as it seems like actual usage is going to run into these failures.

[Tom Cook (tom-k-cook)]

Just to note that I've run across this in actual usage. In particular, gtk's GObject type (`gi._gobject.GObject`) produces this error when meliae's `dump_all_objects` method is called, crashing the Python process.