sagemathgh-39499: Improve sage_getfile by looking at __init__
    
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 sagemath#39369

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] 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

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URL: sagemath#39499
Reported by: user202729
Reviewer(s):

Author: Release Manager

src/sage/misc/sageinspect.py

@@ -1328,6 +1328,12 @@ def sage_getfile(obj):
         if isinstance(obj, functools.partial):
             return sage_getfile(obj.func)
         return sage_getfile(obj.__class__)  # inspect.getabsfile(obj.__class__)
+    else:
+        if hasattr(obj, '__init__'):
+            pos = _extract_embedded_position(_sage_getdoc_unformatted(obj.__init__))
+            if pos is not None:
+                (_, filename, _) = pos
+                return filename
 
     # No go? fall back to inspect.
     try:
@@ -1336,6 +1342,11 @@ def sage_getfile(obj):
         return ''
     for suffix in import_machinery.EXTENSION_SUFFIXES:
         if sourcefile.endswith(suffix):
+            # TODO: the following is incorrect in meson editable install
+            # because the build is out-of-tree,
+            # but as long as either the class or its __init__ method has a
+            # docstring, _sage_getdoc_unformatted should return correct result
+            # see https://github.com/mesonbuild/meson-python/issues/723
             return sourcefile.removesuffix(suffix)+os.path.extsep+'pyx'
     return sourcefile
 
@@ -2356,12 +2367,8 @@ class Element:
         try:
             return inspect.getsourcelines(obj)
         except (OSError, TypeError) as err:
-            try:
-                objinit = obj.__init__
-            except AttributeError:
-                pass
-            else:
-                d = _sage_getdoc_unformatted(objinit)
+            if hasattr(obj, '__init__'):
+                d = _sage_getdoc_unformatted(obj.__init__)
                 pos = _extract_embedded_position(d)
                 if pos is None:
                     if inspect.isclass(obj):