Update decorators.py

Because of the change in the structure of the "collections" module, I change a bit.

src/wrf/decorators.py

@@ -1,6 +1,7 @@
 from __future__ import (absolute_import, division, print_function)
 
-from collections import Iterable, OrderedDict
+from collections import OrderedDict
+from collections.abc import Iterable
 
 import wrapt
 import numpy as np