Two digit years are used/tested in py26 only

acme/acme/client.py

@@ -264,10 +264,10 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
             when = parsedate_tz(retry_after)
             try:
                 year = when[0] # raises TypeError if t is None
-                # Handle two-digit years -- but any webserver that thinks
+                # py26: Handle two-digit years -- but any server that thinks
                 # "retry after 99" means "come back after 1999" is.. deprecated
                 if year >= 0 and year < 100:
-                    when = [year + 2000] + list(when[1:])
+                    when = [year + 2000] + list(when[1:]) #  pragma: no cover
                 tzone = when[-1] if when[-1] else 0
                 # raises ValueError/OverflowError
                 return datetime.datetime(*when[:7]) - datetime.timedelta(tzone)