Support either nose or nose2 as test runner

[musicinmybrain]

See https://fedoraproject.org/wiki/Changes/DeprecateNose for some reasons not to rely on nose.

With this change, either nose (nosetests) or nose2 (nose2) can be used as the test runner, and the original nose does not need to be installed to run the tests.

[codecov]

Codecov Report

Merging #27 (6fcb928) into master (0dea04f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #27   +/-   ##
=======================================
  Coverage   86.84%   86.84%           
=======================================
  Files           3        3           
  Lines         266      266           
=======================================
  Hits          231      231           
  Misses         35       35           

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