Fix error in urllib import with python3

Author: Martin Journois

folium/plugins/terminator.py

@@ -1,11 +1,11 @@
 # -*- coding: utf-8 -*-
 
-import urllib2
+from future.moves.urllib.request import urlopen as _urlopen
 
 from .plugin import Plugin
 
 # As LO.Terminator.js is not served on both HTTP and HTTPS, we need to embed it explicitely into the code.
-_request = urllib2.urlopen("http://rawi.8713187.xyz/joergdietrich/Leaflet.Terminator/master/L.Terminator.js")
+_request = _urlopen("http://rawi.8713187.xyz/joergdietrich/Leaflet.Terminator/master/L.Terminator.js")
 assert _request.getcode()==200, "Error while loading Leaflet.terminator.js"
 _terminator_script = _request.read()