Improve sage_getfile by looking at __init__

[user202729]

Otherwise it will fail with meson editable install on objects like x of type Expression, where the class does not have a docstring but __init__ method have.

The strategy is similar to sage_getsourcelines where __init__ method is looked in.

I choose to implement this instead of the more complex workaround (see the comment below).

This fixes the test that fails in meson editable mode:

2025-02-11T20:13:36.4801998Z **********************************************************************
2025-02-11T20:13:36.4803076Z File "src/sage/misc/sageinspect.py", line 1363, in sage.misc.sageinspect.sage_getfile_relative
2025-02-11T20:13:36.4804104Z Failed example:
2025-02-11T20:13:36.4804750Z     sage_getfile_relative(x)                                                  # needs sage.symbolic
2025-02-11T20:13:36.4808395Z Expected:
2025-02-11T20:13:36.4813350Z     'sage/symbolic/expression.pyx'
2025-02-11T20:13:36.4814244Z Got:
2025-02-11T20:13:36.4815714Z     '/home/runner/work/sage/sage/builddir/src/sage/symbolic/expression.pyx'
2025-02-11T20:13:44.3128543Z **********************************************************************

This may still fail in another case where neither class nor __init__ method has a docstring (in that case ? will fail to get the file in meson editable mode), but that's not in scope I guess. (I left a comment there to explain)

See also #39369

📝 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. (change should be invisible to users not using meson editable so…)

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit f77df2a; changes) is ready! 🎉
This preview will update shortly after each push to this PR.