Applied a patch from dkg@fifthhorseman.net: py3 compatibility: Use a list of dictionary keys

In python3, dict.keys() produces a dict_keys object, not a list.
    Since this code treats it as a list, we'll just be explicit about
    that.
 - Legacy-Id: 15893

ietf/utils/draft.py

@@ -1278,7 +1278,7 @@ def _output(docname, fields, outfile=sys.stdout):
                 outfile.write("%s " % (fields["eventdate"]))
             outfile.write("%s" % (os.path.basename(docname.strip())))
 
-        keys = fields.keys()
+        keys = list(fields.keys())
         keys.sort()
         for key in keys:
             if fields[key] and not key in ["eventdate", ] and not key.startswith("_"):