Call to parent tp_clear() despite @no_gc_clear

[scoder]

@cython.no_gc_clear
cdef class NoClearParent:
     ...

cdef class Child(NoClearParent):
    ...

This fails to C-compile because the child has a tp_clear() implementation that calls the parent class function, but that is not defined.

I think the right thing to do would be to simply not generate the call to the parent class function in tp_clear() (see ModuleNode.py) if the parent has no_gc_clear set.