[3.13] Fix the doctest.testmod() docstring (GH-136675) (GH-136691)

__test__ = None is not supported since Python 2.4.
(cherry picked from commit cb59eae)

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: miss-islington

Lib/doctest.py

@@ -1989,8 +1989,8 @@ def testmod(m=None, name=None, globs=None, verbose=None,
     from module m (or the current module if m is not supplied), starting
     with m.__doc__.
 
-    Also test examples reachable from dict m.__test__ if it exists and is
-    not None.  m.__test__ maps names to functions, classes and strings;
+    Also test examples reachable from dict m.__test__ if it exists.
+    m.__test__ maps names to functions, classes and strings;
     function and class docstrings are tested even if the name is private;
     strings are tested directly, as if they were docstrings.