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.
Bump a few requirements to increase compatibility

The upgraded packages all increase Python 3 compatibility. Bumping was
either necessary because the current release doesn't support Python 3,
or at least advisable.
Use unreleased versions of Compressor and Sorl for Python 3

Let's hope both of them release a new stable version in time for Oscar
0.7.
Introduce requirements_py2.txt for Python 2-only dependencies

Unfortunately detox and transifex-client don't even install under Python
3. Moving them to a seperate file will help with Travis CI.
Update Travis CI and tox to run against Python 3

Travis is still allowed to fail against Python 3.

Also removed the 'install' section which seems to do nothing.