gh-36533: `pkgs/sage-conf_pypi`: Repair after #36400, #36435
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This repairs the installation method described in
https://github.com/sagemath/sage#alternative-installation-using-pypi
broken by changes in #36400, #36435.
We also expand the tests for this distribution in its tox.ini.
Run `(cd pkgs/sage-conf_pypi && tox -v -v -e py311)`
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URL: https://github.com/sagemath/sage/pull/36533
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee
