bpo-37685: Fixed comparisons of datetime.timedelta and datetime.timezone. (GH-14996)

There was a discrepancy between the Python and C implementations.

Add singletons ALWAYS_EQ, LARGEST and SMALLEST in test.support
to test mixed type comparison.
diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 7d62a94..da6a85d 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -356,11 +356,28 @@
 
    Check for presence of docstrings.
 
+
 .. data:: TEST_HTTP_URL
 
    Define the URL of a dedicated HTTP server for the network tests.
 
 
+.. data:: ALWAYS_EQ
+
+   Object that is equal to anything.  Used to test mixed type comparison.
+
+
+.. data:: LARGEST
+
+   Object that is greater than anything (except itself).
+   Used to test mixed type comparison.
+
+
+.. data:: SMALLEST
+
+   Object that is less than anything (except itself).
+   Used to test mixed type comparison.
+
 
 The :mod:`test.support` module defines the following functions: