build: Remove deprecated codecov package

[timmc-edx]

This package was today deleted from PyPI. We already use the GHA version of it, though, so just removing the package should be fine.

Also remove mock from the same file, since it doesn't seem we need it.

Merge checklist:

• [x] Version bumped
• [x] Changelog record added
• [x] Documentation updated (not only docstrings)
• [x] Commits are squashed