Switch to pytest framework

[jonbiemond]

I propose switching to the 3rd party pytest framework from the builtin unittest framework.

The pytest framework has a number of advantages over unittest, and is very popular, used by major libraries such as NumPy. Critically, switching to pytest will not require rewriting any existing tests. Some benefits include:

• The ability to mark tests as slow and skip them for faster development.
• Easier to use, because it does not require that tests inherit from TestCase.
• Detailed and easy to understand failing test summaries.

This change will lower the barrier to writing tests for contributors and reduce the friction in running tests by providing an option to skip slow tests. I hope that these improvements will lead to more tests where appropriate and a more stable library.

I've successfully run pytest over the existing test suite, and would be happy to open a small PR upon consensus.

For more information see the docs: https://docs.pytest.org/en/stable/

[JWock82]

I think this is a great idea. I'd like to keep my focus on some of the engineering/technical parts of Pynite right now. If you want to head this up I will approve the pull requests.