bpo-40204: Add :noindex: in the documentation (GH-21859)

Add :noindex: to duplicated documentation to fix "duplicate object
description" errors.

For example, fix this Sphinx 3 issue:

Doc/library/configparser.rst:1146: WARNING: duplicate object
description of configparser.ConfigParser.optionxform, other instance
in library/configparser, use :noindex: for one of them
diff --git a/Doc/library/difflib.rst b/Doc/library/difflib.rst
index 7a898c2..25e3511 100644
--- a/Doc/library/difflib.rst
+++ b/Doc/library/difflib.rst
@@ -24,6 +24,7 @@
 
 
 .. class:: SequenceMatcher
+   :noindex:
 
    This is a flexible class for comparing pairs of sequences of any type, so long
    as the sequence elements are :term:`hashable`.  The basic algorithm predates, and is a
@@ -651,6 +652,7 @@
 
 
 .. class:: Differ(linejunk=None, charjunk=None)
+   :noindex:
 
    Optional keyword parameters *linejunk* and *charjunk* are for filter functions
    (or ``None``):