`urllib.request.URLopener` is deprecated

Wagon uses [`urllib.request.URLopener`](https://docs.python.org/3/library/urllib.request.html#urllib.request.URLopener), which was deprecated in Python 3.3. Would you accept a patch that uses [`urlopen()`](https://docs.python.org/3/library/urllib.request.html#urllib.request.urlopen) instead? Ta!