TST/BLD : python 3 related issues

It turns out that between 2 and 3 they changed the api for c-extensions.
This means that `ctrans` does not work for py3k out of the box.

 - fixed minor bug in sorting out the configuration files
 - forcibly skip the tests when using python3
Merge pull request #1 from tacaswell/Reciprocal_space

TST/BLD : python 3 related issues