Ditch codecov in favor of code coverage reporting that works

[jeff-cohere]

I'm getting tired of rerunning builds because of code coverage report upload failures, so I'm looking at alternatives:

• https://hynek.me/articles/ditch-codecov-python/
• https://github.com/ewjoachim/coverage-comment-action
• https://nedbatchelder.com/blog/202209/making_a_coverage_badge.html

[jeff-cohere]

Bumping to v4 of the action and making CI indifferent to the upload/reporting process fixes this for now.

[bio-boris]

@jeff-cohere codecov does have some issues, but we found they go away when you add a token.

[jeff-cohere]

Thanks, @bio-boris .