py3k: Correctly handle string input to slicers.

In Python 3, str has the __iter__ method, so just checking for it causes
some invalid iteration through the string characters.

lib/iris/cube.py

@@ -2306,7 +2306,8 @@ def _as_list_of_coords(self, names_or_coords):
         Convert a name, coord, or list of names/coords to a list of coords.
         """
         # If not iterable, convert to list of a single item
-        if not hasattr(names_or_coords, '__iter__'):
+        if (not hasattr(names_or_coords, '__iter__') or
+                isinstance(names_or_coords, str)):
             names_or_coords = [names_or_coords]
 
         coords = []
@@ -2352,7 +2353,8 @@ def slices_over(self, ref_to_slice):
 
         """
         # Required to handle a mix between types.
-        if not hasattr(ref_to_slice, '__iter__'):
+        if (not hasattr(ref_to_slice, '__iter__') or
+                isinstance(ref_to_slice, str)):
             ref_to_slice = [ref_to_slice]
 
         slice_dims = set()
@@ -2412,7 +2414,8 @@ def slices(self, ref_to_slice, ordered=True):
             raise TypeError("'ordered' argument to slices must be boolean.")
 
         # Required to handle a mix between types
-        if not hasattr(ref_to_slice, '__iter__'):
+        if (not hasattr(ref_to_slice, '__iter__') or
+                isinstance(ref_to_slice, str)):
             ref_to_slice = [ref_to_slice]
 
         dim_to_slice = []