Axiom finalization cache does not work on PyPy

[Tristan Seligmann]

`Axiom._fincache`, the code in `Store` that uses it, and the relevant tests rely on objects being deallocated and weakref finalizers being called at very specific times. On PyPy, these events do not occur at quite the same times that the code expects, resulting in errors. I believe it is possible to cope with this by adjusting the way the code uses the cache; while items may still linger in the cache for longer than they would have on CPython, I do not think this should cause any application-level breakage except in application code that is doing extremely unsupported things.