gh-37720: pkgs/sagemath-repl/pyproject.toml.m4: Declare extra 'sphinx'
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As discussed in
https://github.com/sagemath/sage/pull/37589#issuecomment-2030456264, the
Sage documentation system uses Sphinx to format the documentation in the
terminal, for example in the help system.
For the modularization project, a simple fallback for the case of Sphinx
not being present was added.
Here we declare the corresponding "extra" so that users can do `pip
install sagemath-repl[sphinx]`.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/37720
Reported by: Matthias Köppe
Reviewer(s):
