Update urllib2 to python3 version, relying on the already available 'future' package to provide the appropriate alias for use in python2 (#41)

Author: othalan

highcharts/highcharts/highcharts.py

@@ -10,7 +10,7 @@
 import json, uuid
 import re
 import datetime
-import urllib2
+import urllib.request, urllib.error, urllib.parse
 import html
 from collections import Iterable
 from .options import BaseOptions, ChartOptions, ColorAxisOptions, \
@@ -334,7 +334,7 @@ def buildhtmlheader(self):
 
 
         if self.offline:
-            opener = urllib2.build_opener()
+            opener = urllib.request.build_opener()
             opener.addheaders = [('User-Agent', 'Mozilla/5.0')]
 
             self.header_css = [