TST: Add token for codecov

[pllim]

Fix #273 (theoretically).

Need to check run master after merge. I don't think this affects PRs from forks because those don't need token for upload.

[mcara]

even with my approval. Don't know what these required statuses are

[pllim]

OSX job is throttled because of... reasons.

Since it is irrelevant here, I overrode branch protection and merged it. Thanks for the approval!

[pllim]

Usually the list of jobs has Required tags on the right side if they are required to pass before merge is allowed.

Also see https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/about-protected-branches

[pllim]

For completeness, coverage upload is fixed on master. Example log: https://github.com/spacetelescope/spherical_geometry/actions/runs/9100666140/job/25016042691

info - 2024-05-15 18:14:20,573 -- Process Upload complete