[1.6.x] Fixed #21335 -- Use importlib from python3 when using python3

Backport of 3351e94ffa from master.

django/utils/importlib.py

@@ -1,6 +1,8 @@
 # Taken from Python 2.7 with permission from/by the original author.
 import sys
 
+from django.utils import six
+
 def _resolve_name(name, package, level):
     """Return the absolute name of the module to be imported."""
     if not hasattr(package, 'rindex'):
@@ -15,7 +17,10 @@ def _resolve_name(name, package, level):
     return "%s.%s" % (package[:dot], name)
 
 
-def import_module(name, package=None):
+if six.PY3:
+    from importlib import import_module
+else:
+    def import_module(name, package=None):
         """Import a module.
 
         The 'package' argument is required when performing a relative import. It