Enable build of docs with Sphinx 6

[ghisvail]

pydata-sphinx-theme is now compatible with Sphinx 6 since version 0.13.

Changes in this commit:

• Add upper bound on Sphinx as recommended by pydra-sphinx-theme
• Add lower bound on pydra-sphinx-theme to force cache updates if necessary
• Drop lower bound on Sphinx, let pydata-sphinx-theme handle it.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (19b6a94) 88.77% compared to head (a717c5d) 88.77%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #525 +/- ## ======================================= Coverage 88.77% 88.77% ======================================= Files 11 11 Lines 1033 1033 ======================================= Hits 917 917 Misses 116 116 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ReproNim). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ReproNim)

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[kaczmarj]

thank you @ghisvail ! i will test this and update this thread.

[kaczmarj]

it works for me, thanks for the pull request @ghisvail !