Normalize the encoding names for Latin-1 and UTF-8 to
'latin-1' and 'utf-8'.

These are optimized in the Python Unicode implementation
to result in more direct processing, bypassing the codec
registry.

Also see issue11303.
diff --git a/Lib/test/test_nntplib.py b/Lib/test/test_nntplib.py
index 4577ddb..e463e52 100644
--- a/Lib/test/test_nntplib.py
+++ b/Lib/test/test_nntplib.py
@@ -813,7 +813,7 @@
 
     def _check_article_body(self, lines):
         self.assertEqual(len(lines), 4)
-        self.assertEqual(lines[-1].decode('utf8'), "-- Signed by André.")
+        self.assertEqual(lines[-1].decode('utf-8'), "-- Signed by André.")
         self.assertEqual(lines[-2], b"")
         self.assertEqual(lines[-3], b".Here is a dot-starting line.")
         self.assertEqual(lines[-4], b"This is just a test article.")