Use pytest instead of Nose

[dentarg]

I saw the following at https://stackoverflow.com/a/52502420/525616

From what I understand, everyone is moving to pytest - an actively-maintained testing framework.

https://github.com/pytest-dev/pytest/#features

Can run unittest (or trial), nose test suites out of the box;

https://nose.readthedocs.io/en/latest/, https://github.com/nose-devs/nose <- last commit 2016