Consider using weakref.finalize rather than `__del__` to clean up endpoint

[wence-]

As discussed starting here #206 (comment), since #206 we attempt to abort the endpoint when the python endpoint is collected (via a __del__ callback). Since this is not the blessed way of implementing finalizers (that's weakref.finalize) we should consider moving to that instead. However, that comes with some potential complications if the Cython wrappers (which hold C++ objects) are participating in reference cycles. If the ref-cycle is broken in a cython object then it is possible that the deallocation that then occurs leaves dangling references.