Add PEP 612 support to typing_extensions stub (#4814)

Author: gvanrossum

third_party/2and3/typing_extensions.pyi

@@ -103,3 +103,12 @@ class TypeAlias: ...
 class SupportsIndex(Protocol, metaclass=abc.ABCMeta):
     @abc.abstractmethod
     def __index__(self) -> int: ...
+
+# PEP 612 support for Python < 3.9
+if sys.version_info >= (3, 10):
+    from typing import Concatenate as Concatenate, ParamSpec as ParamSpec
+else:
+    class ParamSpec:
+        __name__: str
+        def __init__(self, name: str) -> None: ...
+    Concatenate: _SpecialForm = ...