Use urllib from six module in order to be compatible with Python3

python/servo/command_base.py

@@ -27,7 +27,7 @@ import tarfile
 import zipfile
 from xml.etree.ElementTree import XML
 from servo.util import download_file
-import urllib2
+import six.moves.urllib as urllib
 from bootstrap import check_gstreamer_lib
 
 from mach.decorators import CommandArgument
@@ -506,15 +506,15 @@ class CommandBase(object):
         nightly_date = nightly_date.strip()
         # Fetch the filename to download from the build list
         repository_index = NIGHTLY_REPOSITORY_URL + "?list-type=2&prefix=nightly"
-        req = urllib2.Request(
+        req = urllib.request.Request(
             "{}/{}/{}".format(repository_index, os_prefix, nightly_date))
         try:
-            response = urllib2.urlopen(req).read()
+            response = urllib.request.urlopen(req).read()
             tree = XML(response)
             namespaces = {'ns': tree.tag[1:tree.tag.index('}')]}
             file_to_download = tree.find('ns:Contents', namespaces).find(
                 'ns:Key', namespaces).text
-        except urllib2.URLError as e:
+        except urllib.error.URLError as e:
             print("Could not fetch the available nightly versions from the repository : {}".format(
                 e.reason))
             sys.exit(1)