Replace unmaintained nose with pytest

[hugovk]

Fixes #405.

The nightly (Python 3.9) is failing because Nose doesn't support it and is no longer maintained: nose-devs/nose#1099.

Nose has been in maintenance mode for the past several years and will likely cease without a new person/team to take over maintainership. New projects should consider using Nose2, py.test, or just plain unittest/unittest2.

https://nose.readthedocs.io/en/latest/

nose2 may or may not be a good fit for your project.

If you are new to python testing, we encourage you to also consider pytest, a popular testing framework.

https://docs.nose2.io/en/latest/

I also recommend pytest.

[coveralls]

Coverage increased (+0.3%) to 30.377% when pulling d042274 on hugovk:replace-nose-with-pytest into 3a8674a on sixohsix:master.

[RouxRC]

Thanks for the update Hugo !

[hugovk]

You're welcome!