The Codecov bash uploader that the CI is using is deprecated and will stop working on Feb 1, 2022, with brownouts before then.

https://about.codecov.io/blog/introducing-codecovs-new-uploader/

We should update to their new uploader.

The relevant file is https://github.com/pytest-dev/pytest/blob/5fc7b21391c959bb5a54174d9084ad55131b8fdb/scripts/upload-coverage.sh

Called in CI from here:

Note: the scripts/upload-coverage.sh script itself can stay in bash; just the codecov file it downloads should be changed to the new one.