gh-36979: Fix `sage-get-system-packages install-requires` after #29665
Currently broken (since #29665):
```
$ build/bin/sage-get-system-packages install-requires zipp
$ build/bin/sage-get-system-packages install-requires-toml zipp
```
This causes `pkgs/sagemath-standard/pyproject.toml` to be broken; as
noted in
https://github.com/sagemath/sage/pull/36964#issuecomment-1871672884
The correct output:
```
$ build/bin/sage-get-system-packages install-requires zipp
zipp >=0.5.2
$ build/bin/sage-get-system-packages install-requires-toml zipp
'zipp >=0.5.2',
```
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URL: https://github.com/sagemath/sage/pull/36979
Reported by: Matthias Köppe
Reviewer(s): Michael Orlitzky
