Undo the removal. Guido mentioned that the encoding name is in active
by some email headers.
diff --git a/Lib/encodings/aliases.py b/Lib/encodings/aliases.py
index c1181eb..6620c2e 100644
--- a/Lib/encodings/aliases.py
+++ b/Lib/encodings/aliases.py
@@ -26,6 +26,7 @@
     # ascii codec
     '646'                : 'ascii',
     'ansi_x3.4_1968'     : 'ascii',
+    'ansi_x3_4_1968'     : 'ascii', # some email headers use this non-standard name
     'ansi_x3.4_1986'     : 'ascii',
     'cp367'              : 'ascii',
     'csascii'            : 'ascii',