Make it more clear that setUpClass runs before each class, not "class run" (GH-8844)

(cherry picked from commit c33bb5d)

Co-authored-by: Ville Skyttä <ville.skytta@iki.fi>

Author: miss-islington

Doc/library/unittest.rst

@@ -730,7 +730,7 @@ Test cases
 
    .. method:: setUpClass()
 
-      A class method called before tests in an individual class run.
+      A class method called before tests in an individual class are run.
       ``setUpClass`` is called with the class as the only argument
       and must be decorated as a :func:`classmethod`::