Remove codecov, replace with python-coverage-comment-action

[nedbat]

Description: Describe in a couple of sentences what this PR adds

ISSUE: Link to GitHub issue

Dependencies: dependencies on other outstanding PRs, issues, etc.

Merge deadline: List merge deadline (if any)

Installation instructions: List any non-trivial installation instructions.

Testing instructions:

1. Open page A
2. Do thing B
3. Expect C to happen
4. If D happened instead - check failed.

Reviewers:

• [ ] tag reviewer
• [ ] tag reviewer

Merge checklist:

• [ ] All reviewers approved
• [ ] CI build is green
• [ ] Version bumped
• [ ] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed

Post merge:

• [ ] Create a tag
• [ ] Check new version is pushed to PyPI after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)

Author concerns: List any concerns about this PR - inelegant solutions, hacks, quick-and-dirty implementations, concerns about migrations, etc.

[nedbat]

@dianakhuang @timmc-edx @robrap Feel free to do something with this if you like.

[robrap]

Thanks @nedbat. Let us know if there is any other status you could report, other than just looking at the code.