VersionedUnpickler depends on private implementation details of _Unpickler

[aaronayres35]

As described in this comment chain: #199 (comment)

The apptools.persistence.versioned_unpickler.NewUnpickler class currently depends on an attribute dispatch defined on the _Unpickler class from pickle, which is not intended to be public. Further this is forcing VersionedUnpickler to continue using _Unpickler not the Unpicklerwhich comes from the faster _pickle (see here: https://github.com/python/cpython/blob/aa01011003bb855cd52abfd49f2443446590d913/Lib/pickle.py#L1772-L1787).

Ultimately, we should remove the dependency on dispatch which is part of a private interface, and evolve VersionedUnpickler to be able to use Unpickler / not be tied to _Unpickler.