Update ud_opcode.py to also support Python 3.x

Hit this on Arch, which uses Python 3 as default interpreter. The scripts should either call `/usr/bin/python2` instead of just `/usr/bin/python` or this script must be updated. The proposed change(s) in particular:

    * `dict.iteritems()` has been replaced by `dict.items()`, which returns a lazy view instead. `itemslist()` resolves this.