bpo-32226: PEP 560: improve typing module (#4906)

This PR re-designs the internal typing API using the new PEP 560 features.
However, there are only few minor changes in the public API.

Lib/dataclasses.py

@@ -389,7 +389,8 @@ def _get_field(cls, a_name, a_type):
     if typing is not None:
         # This test uses a typing internal class, but it's the best
         #  way to test if this is a ClassVar.
-        if type(a_type) is typing._ClassVar:
+        if (type(a_type) is typing._GenericAlias and
+                a_type.__origin__ is typing.ClassVar):
             # This field is a ClassVar, so it's not a field.
             f._field_type = _FIELD_CLASSVAR
 

Lib/test/libregrtest/refleak.py

@@ -135,11 +135,6 @@ def dash_R_cleanup(fs, ps, pic, zdc, abcs):
     # Clear ABC registries, restoring previously saved ABC registries.
     abs_classes = [getattr(collections.abc, a) for a in collections.abc.__all__]
     abs_classes = filter(isabstract, abs_classes)
-    if 'typing' in sys.modules:
-        t = sys.modules['typing']
-        # These classes require special treatment because they do not appear
-        # in direct subclasses of collections.abc classes
-        abs_classes = list(abs_classes) + [t.ChainMap, t.Counter, t.DefaultDict]
     for abc in abs_classes:
         for obj in abc.__subclasses__() + [abc]:
             obj._abc_registry = abcs.get(obj, WeakSet()).copy()

Lib/test/test_pydoc.py

@@ -827,7 +827,7 @@ class TestDescriptions(unittest.TestCase):
                          'f\x08fo\x08oo\x08o(data: List[Any], x: int)'
                          ' -> Iterator[Tuple[int, Any]]')
         self.assertEqual(pydoc.render_doc(C).splitlines()[2],
-                         'class C\x08C(typing.Mapping)')
+                         'class C\x08C(collections.abc.Mapping, typing.Generic)')
 
     def test_builtin(self):
         for name in ('str', 'str.translate', 'builtins.str',