The keys are immutable anyway so the instances of PyDict could manage
a opaque value (in fact, a counter) that changes every time a new
value is written to any key. Once we get a reference out of the dict,
we can can do very fast lookups by passing the key, the reference we
know from the last lookup and our last state. The lookup returns a new
reference and the new state.
If the dict has not changed, the state doesnt change and the reference
is simply taken from the passed value passed to the lookup. That way
the code remains the same no matter if the dict has changed or not.