Replace urllib with urllib2, fixing proxy support

Calling urllib.open in version.py caused pywikibot to halt indefinately
on network problems, such as a firewall, as urllib v1 has no socket timeout.

wikipedia.py now installs MyURLopener (which has proxy support) as the
default urllib2 opener, and version.py implicitly uses this opener.

Remove unused imports of urllib & urllib2.

Change-Id: Ic36852fc6c5657481d81a5117b22842868ad2565
Merge "Replace urllib with urllib2, fixing proxy support"