bpo-36019: Use pythontest.net instead of example.com in network tests (GH-11941)

(cherry picked from commit a40681dd5db8deaf05a635eecb91498dac882aa4)

Co-authored-by: Stéphane Wirtel <stephane@wirtel.be>
diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 95d7f54..de79cdf 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -356,6 +356,10 @@
 
    Check for presence of docstrings.
 
+.. data:: TEST_HTTP_URL
+
+   Define the URL of a dedicated HTTP server for the network tests.
+
 
 
 The :mod:`test.support` module defines the following functions: