switch docs to flit

CHANGES.rst

@@ -309,7 +309,7 @@ Released 2022-03-28
     or ``AppContext.g`` instead. :issue:`3898`
 -   ``copy_current_request_context`` can decorate async functions.
     :pr:`4303`
--   The CLI uses ``importlib.metadata`` instead of ``setuptools`` to
+-   The CLI uses ``importlib.metadata`` instead of ``pkg_resources`` to
     load command entry points. :issue:`4419`
 -   Overriding ``FlaskClient.open`` will not cause an error on redirect.
     :issue:`3396`