gh-36751: Switch sage-conf to modern editable install
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This modernization eliminates some deprecation warnings during
installation. It may help solve the problem of the disappearing sage-
conf (https://groups.google.com/g/sage-
release/c/J6mGYH56FKA/m/m9yHivCWAgAJ, https://groups.google.com/g/sage-
release/c/dvPti2UkyjQ/m/2jeUZROzAwAJ, https://groups.google.com/g/sage-
release/c/DeqhtAgi2es/m/DYq13owvAQAJ).
No change to how sagelib is installed; this will be taken care of
separately (#34209).
In combination with #36562, which adds `pyproject.toml` for sage-
docbuild, sage-setup, sage-sws2rst, also these distributions are
switched to the modern editable install.
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URL: https://github.com/sagemath/sage/pull/36751
Reported by: Matthias Köppe
Reviewer(s): John H. Palmieri
