Separate test requirements to new file and enable suite to run without any optional requirements
This MR separates the test requirements to a separate requirements file, and then updates the suite itself to gracefully skip any tests that require optional requirements when they aren't available.
I've added a CI job to run the following basic test:
pip install pesummary-*.tar.*
pip install -r test-requirements.txt
pytest pesummary/
which should validate the 'optionalness' of the requirements wherever they are.