unittest.mock.__version__ was removed in 3.9 (#10056)

See the first entry in https://docs.python.org/3/whatsnew/3.9.html#removed

Author: FasterSpeeding

stdlib/unittest/mock.pyi

@@ -47,7 +47,8 @@ else:
         "seal",
     )
 
-__version__: Final[str]
+if sys.version_info < (3, 9):
+    __version__: Final[str]
 
 FILTER_DIR: Any