Currently, if a dependency of a Cython module is not available, then meson will simply ignore the Cython module. If then a user (or some other parts of sage) tries to import the Cython module, this will fail with a ModuleNotFound error. This is problematic, since there is no indication of the underlying reason (the missing dependency) why the import is not succesful.

This PR proposes to fix this by installing a small dummy python module in place of the original Cython module when the dependencies are not avaiable. This python module then raises a FeatureNotPresent error when one tries to use it.
(In the future, this error may even say what dependency is missing; but this is not yet implemented and needs a few small changes upstream in meson first.)

Instead of raising the FeatureNotPresent error directly upon import, the error is only raised once one actually tries to call the imported method or class. This is done so that the many imports in .all are still "working" and don't raise exceptions. So in short:

# This will never raise an exception
from sage.module.with.missing.dependency import some_fct

# but this will
some_fct("Hello")

This added flexibilty should make it possible to react very dynamically and gracefully when dependencies are missing. This is especially important for Windows, where a bunch of dependencies are just not available (and this will also not change in the near future); see #38872 (comment).

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies