iat being set to null is not handled properly

[Colin-b]

If iat is provided, but the value is null (or None once deserialized), a TypeError is raised instead of a JWTClaimsError.

I think the following piece of code should handle TypeError as well as ValueError:

https://github.com/mpdavis/python-jose/blob/58e543ea5ea225961e43e9cc2781165748478a0f/jose/jwt.py#L256