Use the standard lib `tomllib`, or default to `tomli` if not available

[mwouts]

Closes #1195

[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:

pip install git+https://github.com/mwouts/jupytext.git@use_tomllib_or_tomli

(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 97.73%. Comparing base (642c709) to head (e9c56ab).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1200 +/- ## ======================================= Coverage 97.73% 97.73% ======================================= Files 29 29 Lines 4463 4464 +1 ======================================= + Hits 4362 4363 +1 Misses 101 101 ``` | [Flag](https://app.codecov.io/gh/mwouts/jupytext/pull/1200/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/1200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `75.17% <75.00%> (+0.14%)` | :arrow_up: | | [functional](https://app.codecov.io/gh/mwouts/jupytext/pull/1200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `88.51% <75.00%> (+0.02%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/mwouts/jupytext/pull/1200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `77.28% <100.00%> (+<0.01%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/mwouts/jupytext/pull/1200/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Marc+Wouts) | `66.56% <87.50%> (+<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=Marc+Wouts#carryforward-flags-in-the-pull-request-comment) to find out more.

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