Switch to pytest framework for testing

[jonbiemond]

Add the pytest library for running tests as detailed in issue #255

• Update requirements.txt to included pytest
• Run tests in the test-and-build workflow with pytest instead of unittest
• Add instructions to CONTRIBUTING.md on how to run tests with pytest

Resolves #225

[JWock82]

This is great. I just want to be sure the test reports generated are ready to plug in to codecov before I pull it. I plan to have a closer look at it this next week.