setup.py says we use nose for testing, but we don't

[michaelosthege]

Someone noticed a few weeks back that there's also a dependency on nose in one of the requirements file.

We changed to pytest and I'm not aware of any nose usage anywhere, so let's take it out.

[ColCarroll]

To add historical context, this was a dependency through theano, which for reasons required it to even import the project. If pymc-theano removes that, then this should be straightforward.