Python reference cycles through attributes are now collected

Storing an attribute on a wrapped VTK object no longer risks leaking that object for the lifetime of the process. The Python wrapper’s tp_traverse now reports the instance dictionary to CPython’s cyclic garbage collector, so a cycle that passes through an attribute – for example a cached helper that holds the object it was created from – is found and freed:

import gc
from vtkmodules.vtkCommonCore import vtkPoints

class Accessor:
    def __init__(self, obj):
        self.obj = obj

points = vtkPoints()
points.accessor = Accessor(points)  # cycle: points -> __dict__ -> accessor -> points
del points
gc.collect()                        # frees both; previously freed neither

Previously the collector was told that the wrapper referred to nothing but its observers, so such a cycle looked externally referenced and neither the wrapper nor the C++ object it held was ever released. Code that worked around this by holding a vtkWeakReference remains correct.