Exclude broken typing-extensions version + fix import

The ipynb code was assuming that typing-extensions would always be
available, but that's not the case! There's an environment marker
on the requirement meaning it won't get installed on 3.10 or higher.
The test suite didn't catch this issue since aiohttp pulls in
typing-extensions unconditionally.

CHANGES.md

@@ -14,6 +14,7 @@
 - Parsing support has been added for unparenthesized walruses in set literals, set
   comprehensions, and indices (#2447).
 - Pin `setuptools-scm` build-time dependency version (#2457)
+- Exclude typing-extensions version 3.10.0.1 due to it being broken on 3.10 (#2460)
 
 ### _Blackd_
 

setup.py

@@ -79,7 +79,10 @@ def get_long_description() -> str:
         "regex>=2020.1.8",
         "pathspec>=0.9.0, <1",
         "dataclasses>=0.6; python_version < '3.7'",
-        "typing_extensions>=3.10.0.0; python_version < '3.10'",
+        "typing_extensions>=3.10.0.0",
+        # 3.10.0.1 is broken on at least 3.10,
+        # https://github.com/python/typing/issues/865
+        "typing_extensions!=3.10.0.1; python_version >= '3.10'",
         "mypy_extensions>=0.4.3",
     ],
     extras_require={

src/black/handle_ipynb_magics.py

@@ -1,15 +1,19 @@
 """Functions to process IPython magics with."""
+
 from functools import lru_cache
 import dataclasses
 import ast
-from typing import Dict
+from typing import Dict, List, Tuple, Optional
 
 import secrets
-from typing import List, Tuple
+import sys
 import collections
 
-from typing import Optional
-from typing_extensions import TypeGuard
+if sys.version_info >= (3, 10):
+    from typing import TypeGuard
+else:
+    from typing_extensions import TypeGuard
+
 from black.report import NothingChanged
 from black.output import out