Add code coverage report for each PR, block PRs if coverage decreases

[truskovskiyk]

Reference:

https://coverage.readthedocs.io/en/7.4.4/
https://pypi.org/project/pytest-cov/

https://github.com/marketplace/actions/code-coverage-summary
https://github.com/marketplace/actions/code-coverage-report-difference

[benjaminye]

Addressed in #184

• Set a hard floor of 80%