Switch to nose2

[tovrstra]

https://github.com/nose-devs/nose2

@matt-chan The version of nose that we use at the moment is no longer actively developed and will likely start suffering from bitrot in the near future. (See note to users on the web page: https://nose.readthedocs.io/en/latest/) We should either switch to unittest2 or nose2. The second option seems to be better: more plugins, including coverage. It is also easier to write tests for nose2: no need for classes, simple asserts are fine.