src/doc/en/developer/coding_in_python.rst: Mention typing_extensions

Author: Matthias Koeppe

src/doc/en/developer/coding_in_python.rst

@@ -33,8 +33,8 @@ using one of two mechanisms:
   Sage library code that uses type annotations should include this
   ``__future__`` import and follow PEP 563.
 
-- The Sage distribution includes the backport packages ``importlib_metadata``
-  and ``importlib_resources``.
+- The Sage distribution includes the backport packages ``importlib_metadata``,
+  ``importlib_resources``, and ``typing_extensions``.
 
 Meta-ticket :trac:`29756` keeps track of newer Python features and serves
 as a starting point for discussions on how to make use of them in the