be used to declare covariant or contravariant
    generic types.  These keyword arguments are valid, but their actual semantics
    are yet to be decided.  See PEP 612 for details.

    Parameter specification variables can be introspected. e.g.:

       P.__name__ == 'P'
       P.__bound__ == None
       P.__covariant__ == False
       P.__contravariant__ == False

    Note that only parameter specification variables defined in global scope can
    be pickled.
    c