Fix test errors: codecov secret and missing package for docs

[adeliegorce]

This PR adds two elements that were missing for the tests to pass:

• Adds a secret for codecov. Although it is not required for public repos, uploads often fail when you don't have one. If you don't set one, it uses like a backup public token which has a rate limit (@steven-murray)
• Adds the mock package as a requirement for the documentation: building readthedocs is failing without (called in conf.py)

[adeliegorce]

To whoever reviews this PR, a question: why is there a list of requirements for docs in setup.cfg and a docs/requirements.txt file? it seems like the latter overruns the former? thanks

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (fac41f1) 95.99% compared to head (7f4d549) 95.99%. Report is 7 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #394 +/- ## ======================================= Coverage 95.99% 95.99% ======================================= Files 17 17 Lines 6113 6118 +5 ======================================= + Hits 5868 5873 +5 Misses 245 245 ``` | [Flag](https://app.codecov.io/gh/HERA-Team/hera_pspec/pull/394/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HERA-Team) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/HERA-Team/hera_pspec/pull/394/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HERA-Team) | `95.99% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HERA-Team#carryforward-flags-in-the-pull-request-comment) to find out more.

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[adeliegorce]

Hi @steven-murray, thanks for the review. I have made the requested changes apart from switching to codecov-actions@v4 as there is a regression issue. Let's keep it in mind (or create an issue related to this) and do the update in a couple months once things are more stable.