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.