Require sphinx<8 in tests

[mwouts]

Fixes: the rst2md tests fail in the CI #1266

[github-actions[bot]]

Thank you for making this pull request.

Did you know? You can try it on Binder: or .

Also, the version of Jupytext developed in this PR can be installed with pip:

HATCH_BUILD_HOOKS_ENABLE=true pip install git+https://github.com/mwouts/jupytext.git@fix_rst2md_in_tests

(this requires nodejs, see more at Developing Jupytext)

[codecov[bot]]

Codecov Report

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

Project coverage is 97.00%. Comparing base (6089897) to head (7f2f49b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1268 +/- ## ========================================== + Coverage 95.93% 97.00% +1.07% ========================================== Files 29 29 Lines 4476 4479 +3 ========================================== + Hits 4294 4345 +51 + Misses 182 134 -48 ``` | [Flag](https://app.codecov.io/gh/mwouts/jupytext/pull/1268/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | Coverage Δ | | |---|---|---| | [external](https://app.codecov.io/gh/mwouts/jupytext/pull/1268/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `75.02% <ø> (?)` | | | [integration](https://app.codecov.io/gh/mwouts/jupytext/pull/1268/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `77.25% <ø> (?)` | | | [unit](https://app.codecov.io/gh/mwouts/jupytext/pull/1268/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `66.56% <ø> (?)` | | 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=Marc+Wouts#carryforward-flags-in-the-pull-request-comment) to find out more.

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