Fix more doctests in meson_editable install

[user202729]

Fix more bugs uncovered by #39369 .

Add a feature flag meson_editable, and put doctests behind this flag accordingly.

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[tobiasdiez]

Thanks!

[user202729]

That said, do you have any better way than using type(sage.__loader__).__module__ == '_sagemath_editable_loader' to check for the existence of the feature? Given the underscore it is quite obvious that it's meant to be an internal name.

[tobiasdiez]

That said, do you have any better way than using type(sage.__loader__).__module__ == '_sagemath_editable_loader' to check for the existence of the feature? Given the underscore it is quite obvious that it's meant to be an internal name.

No not really. But in my opinion it's a good test since the loader is the most obvious indication that one is using meson in editable mode.