Undo Python 3 work on csv_utils.py

It's asking for a world of pain to get that working reliably with both
Python 2 and 3.
Add CSV wrappers that abstract away differences between Python 2 and 3

Code gratefully taken from http://python3porting.com/problems.html
Use new wrapper classes for StockImporter and CatalogueImporter

They're apparently deprecated, but currently used to build the sandbox.