[Dipy] Heads up: rebase needed

Hey everyone, I just merged in https://github.com/nipy/dipy/pull/1652. If you are currently working on an active branch, you will probably need to rebase your work on top of this, and if you touched any of the tests, you are likely to run into some merge conflicts. Pleas let us know (here, or on your PR) if you are running into any issues and if you need any guidance in resolving conflicts. The switch to pytest will hopefully be invisible to most of our users and even developers, but is a great step forward. Congratulations to Serge for all his hard work on the PR implementing it. Cheers, Ariel