Stop using deprecated urllib.request.URLopener

[barneygale]

This was deprecated in Python 3.3.

Fixes #157

[davidfstr]

wagon no longer imports on Python 3.14+ due to this problem.

Here's how I got wagon to be usable on Python 3.14:

WAGON_PATH=/usr/local/lib/python3.14/site-packages/wagon.py
sed -i 's/from urllib.request import URLopener/from urllib.request import urlopen/' $WAGON_PATH
sed -i 's/from urllib import urlopen/from urllib.request import urlopen/' $WAGON_PATH