Put coord_categorisation back to master

Author: znicholls

lib/iris/coord_categorisation.py

@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2019, Met Office
+# (C) British Crown Copyright 2010 - 2018, Met Office
 #
 # This file is part of Iris.
 #
@@ -32,7 +32,7 @@
 import six
 
 import calendar
-from collections import Counter
+import collections
 
 import numpy as np
 
@@ -246,7 +246,7 @@ def _validate_seasons(seasons):
     None otherwise.
 
     """
-    c = Counter()
+    c = collections.Counter()
     for season in seasons:
         c.update(_months_in_season(season))
     # Make a list of months that are not present...