Python3 whitespace changes

These changes are related to the fixes provided by [futurize](http://python-future.org/) and are mostly just spaces after commas. This is not a straight run of the `lib2to3.fixes.fix_ws_comma` fixer, because I'm fairly certain for commas around `:`, you specify spacing which 2to2 undoes.
Having the comment between the statement and its output splits up the flow and makes it harder to follow what's going on.

How about:
```
            >>> fname = iris.sample_data_path('air_temp.pp')
            >>> cube = iris.load_cube(fname, 'air_temperature')
            >>> # cube.data does not yet have a value.
            ...
            >>> print cube.shape
            (73, 96)
            >>> # cube.data still does not have a value.
            ...
            >>> cube = cube[:10, :20]
            >>> # cube.data still does not have a value.
            ...
            >>> data = cube.data
            >>> # Only now is the data loaded.
            ...
            >>> print data.shape
            (10, 20)
```