🧪 Add CI testing support for Python 3.13

[hugovk]

The Python 3.13 release candidate is out now! :rocket:

The Release Manager has issued a call to action:

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.13 compatibilities during this phase, and where necessary publish Python 3.13 wheels on PyPI to be ready for the final release of 3.13.0. Any binary wheels built against Python 3.13.0rc1 will work with future versions of Python 3.13. As always, report any issues to the Python bug tracker.

[codecov[bot]]

Codecov Report

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

Project coverage is 90.30%. Comparing base (ce37be5) to head (737e4b4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #955 +/- ## ======================================= Coverage 90.30% 90.30% ======================================= Files 24 24 Lines 3507 3507 ======================================= Hits 3167 3167 Misses 340 340 ``` | [Flag](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/955/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/955/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `90.30% <ø> (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=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more.

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