Slice's members start, stop and step can
be arbitrary python objects, which means
that slices can be easily included in
reference cycles (see the test in the
patch).
The patch includes the necessary changes
to enable support for the gc. Since slices
are immutable, there should be no need for
a tp_clear method.
I also changed the slice_dealloc function
to use Py_CLEAR macros instead of Py_DECREF.

I noticed that PySlice_GetIndices is still
missing support for types with a __index__
method. I don't know if that is intentional
or not.