isinstance checks don't work on data types

[pipermerriam]

What was wrong?

This code returns that the pk is not an instance of PrivateKey. It should return True

In [141]: from eth_keys import keys

In [142]: pk = keys.PrivateKey(b'\x01' * 32)

In [143]: isinstance(pk, keys.PrivateKey)
Out[143]: False

How can it be fixed?

Need to un-tangle the dynamic class generation that occurs in the backends in such a way that we still get a friendly backend API.