Drop reference to typing_extensions

It has been unused since Python 3.10 support was removed by [1].

[1] e6867961c9

Change-Id: Iaefaaac9333572de4a4fd1c185b6b122f3466267
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -86,4 +86,3 @@ exclude = .venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 import_exceptions =
   collections.abc
   typing
-  typing_extensions