bpo-37045: PEP 591: Add final qualifiers to typing module (GH-13571)

The implementation is straightforward, it just mimics `ClassVar` (since the latter is also a name/access qualifier, not really a type). Also it is essentially copied from `typing_extensions`.

Lib/test/test_typing.py

@@ -12,7 +12,7 @@ from typing import T, KT, VT  # Not in __all__.
 from typing import Union, Optional
 from typing import Tuple, List, MutableMapping
 from typing import Callable
-from typing import Generic, ClassVar
+from typing import Generic, ClassVar, Final, final
 from typing import cast
 from typing import get_type_hints
 from typing import no_type_check, no_type_check_decorator
@@ -1438,6 +1438,53 @@ class ClassVarTests(BaseTestCase):
             issubclass(int, ClassVar)
 
 
+class FinalTests(BaseTestCase):
+
+    def test_basics(self):
+        Final[int]  # OK
+        with self.assertRaises(TypeError):
+            Final[1]
+        with self.assertRaises(TypeError):
+            Final[int, str]
+        with self.assertRaises(TypeError):
+            Final[int][str]
+        with self.assertRaises(TypeError):
+            Optional[Final[int]]
+
+    def test_repr(self):
+        self.assertEqual(repr(Final), 'typing.Final')
+        cv = Final[int]
+        self.assertEqual(repr(cv), 'typing.Final[int]')
+        cv = Final[Employee]
+        self.assertEqual(repr(cv), 'typing.Final[%s.Employee]' % __name__)
+
+    def test_cannot_subclass(self):
+        with self.assertRaises(TypeError):
+            class C(type(Final)):
+                pass
+        with self.assertRaises(TypeError):
+            class C(type(Final[int])):
+                pass
+
+    def test_cannot_init(self):
+        with self.assertRaises(TypeError):
+            Final()
+        with self.assertRaises(TypeError):
+            type(Final)()
+        with self.assertRaises(TypeError):
+            type(Final[Optional[int]])()
+
+    def test_no_isinstance(self):
+        with self.assertRaises(TypeError):
+            isinstance(1, Final[int])
+        with self.assertRaises(TypeError):
+            issubclass(int, Final)
+
+    def test_final_unmodified(self):
+        def func(x): ...
+        self.assertIs(func, final(func))
+
+
 class CastTests(BaseTestCase):
 
     def test_basics(self):