Sphinx-substitution-extensions

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- Add support for MyST.
Thanks to Václav Votípka (eNcacz) for the contribution.

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- Bump the minimum supported version of Sphinx to 7.2.0.
- Bump the minimum supported version of docutils to 0.19.
- ``sphinx-prompt`` is no longer an optional dependency, meaning you can remove the ``[prompt]`` extras dependency specification.
- Remove the need to specify the ``sphinx-prompt`` extension in ``conf.py`` in order to use the ``prompt`` directive.
- Support Python 3.12
- Drop support for Python 3.9

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- Breaking change: The required Sphinx version is at least 4.0.
- ``sphinx-prompt`` is now an optional dependency.
Thanks go to dgarcia360 for this change.

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- Ensure non-lower-case replacements can also be substituted in the inline substitution code role.

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- Ensure non-lower-case replacements can also be substituted.
Thanks go to Julian for this change.