🔧 Update mypy to use sphinx v7.3

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.14%. Comparing base (167c902) to head (9ce2d92).

Files	Patch %	Lines
myst_parser/sphinx_ext/myst_refs.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #917 +/- ## ======================================= Coverage 90.14% 90.14% ======================================= Files 24 24 Lines 3400 3400 ======================================= Hits 3065 3065 Misses 335 335 ``` | [Flag](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/917/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/917/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `90.14% <66.66%> (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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