bpo-44098: Drop ParamSpec from most ``__parameters__`` in typing generics (GH-26013)

Added two new attributes to ``_GenericAlias``: 
* ``_typevar_types``, a single type or tuple of types indicating what types are treated as a ``TypeVar``. Used for ``isinstance`` checks.
* ``_paramspec_tvars ``, a boolean flag which guards special behavior for dealing with ``ParamSpec``. Setting it to ``True`` means this  class deals with ``ParamSpec``.

Automerge-Triggered-By: GH:gvanrossum