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

Lib/test/test_typing.py

@@ -4359,6 +4359,31 @@ class ParamSpecTests(BaseTestCase):
         self.assertEqual(C1[int, str], Callable[[int], str])
         self.assertEqual(C1[[int, str, dict], float], Callable[[int, str, dict], float])
 
+    def test_no_paramspec_in__parameters__(self):
+        # ParamSpec should not be found in __parameters__
+        # of generics. Usages outside Callable, Concatenate
+        # and Generic are invalid.
+        T = TypeVar("T")
+        P = ParamSpec("P")
+        self.assertNotIn(P, List[P].__parameters__)
+        self.assertIn(T, Tuple[T, P].__parameters__)
+
+        # Test for consistency with builtin generics.
+        self.assertNotIn(P, list[P].__parameters__)
+        self.assertIn(T, tuple[T, P].__parameters__)
+
+    def test_paramspec_in_nested_generics(self):
+        # Although ParamSpec should not be found in __parameters__ of most
+        # generics, they probably should be found when nested in
+        # a valid location.
+        T = TypeVar("T")
+        P = ParamSpec("P")
+        C1 = Callable[P, T]
+        G1 = List[C1]
+        G2 = list[C1]
+        self.assertEqual(G1.__parameters__, (P, T))
+        self.assertEqual(G2.__parameters__, (P, T))
+
 
 class ConcatenateTests(BaseTestCase):
     def test_basics(self):