Make it clear that the msg argument to assertRaises is keyword-only. (GH-9670)

(cherry picked from commit be4e5b8)

Co-authored-by: Benjamin Peterson <benjamin@python.org>

Author: miss-islington

Doc/library/unittest.rst

@@ -944,7 +944,7 @@ Test cases
    +---------------------------------------------------------+--------------------------------------+------------+
 
    .. method:: assertRaises(exception, callable, *args, **kwds)
-               assertRaises(exception, msg=None)
+               assertRaises(exception, *, msg=None)
 
       Test that an exception is raised when *callable* is called with any
       positional or keyword arguments that are also passed to