Add allowlist entry

Author: srittau

stdlib/@tests/stubtest_allowlists/py39.txt

@@ -44,6 +44,10 @@ tempfile.SpooledTemporaryFile.writable
 
 tkinter.Tk.split  # Exists at runtime, but missing from stubs
 
+# Will always raise. Not included to avoid type checkers inferring that
+# TypeAliasType instances are callable.
+typing_extensions.Sentinel.__call__
+
 
 # =======
 # <= 3.11