Add combined coverage reporting

Python requires a lot of environments for effective testing. The python pipeline only formally supports testing for a single Python version on a single architecture.

• Create two new jobs:
  • python-coverage-pytest: Run a parallel matrix of coverage run -m pytest for each supported platform and Python version.
    • Save coverage in a file named .coverage.<job-name>.<platform>.<python-version> or similar to prevent clobbering.
    • Save as an artifact.
  • python-coverage-combine: Run coverage combine and friends to compute final coverage results and render reports.
    • Use dependencies keyword so that only pytest results are included in coverage report.
• Wire coverage reporting test into project CI/CD.
• Update README.