Update unittest framework to pytest

[danodonovan]

Suggestion; to update the unit tests from nose to pytest.

Motivation;

• testing fails for python > 3.9 (though the library works fine)
• updating precisely to support more recent python difficult
• blocks further dev work

Reasons for pytest choice;

• nose does not support recent python versions and looks to be un-maintained
  • patching nose would be difficult
  • forks of nose (nose2 may have similar problems)
  • nose is not a popular test framework any longer
• pytest is well supported, quite popular and currently the closer to a standard test framework

Other considerations;

• pytest isn't a drop-in replacement for nose
• requires some reformatting and renaming of tests

CF #21 as a DRAFT implementation of this change

NB Apologies - I had missed the CONTRIBUTING note

[mwilliamson]

Thanks for the suggestion! master is now using pytest.