sagemathgh-39424: Fix bug with sage_getfile in meson editable install
    
Previously, the following test fails with meson editable install

```
2025-01-28T04:27:13.4765580Z File "src/sage/misc/cachefunc.pyx", line
881, in sage.misc.cachefunc.CachedFunction._instancedoc_
2025-01-28T04:27:13.4766297Z Failed example:
2025-01-28T04:27:13.4766651Z
os.path.exists(sage_getfile(I.groebner_basis))
2025-01-28T04:27:13.4767084Z Expected:
2025-01-28T04:27:13.4767330Z     True
2025-01-28T04:27:13.4767571Z Got:
2025-01-28T04:27:13.4767808Z     False
```

See https://github.com/sagemath/sage/actions/runs/13003203795/job/362655
39648 .

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- [ ] I have created tests covering the changes.  (can't really test,
but see sagemath#39369)
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URL: sagemath#39424
Reported by: user202729
Reviewer(s): Tobias Diez

Author: Release Manager

src/sage/misc/sageinspect.py

@@ -276,8 +276,10 @@ def _extract_embedded_position(docstring):
         from sage.misc.temporary_file import spyx_tmp
         if raw_filename.startswith('sage/'):
             import sage
-            try_filenames = [os.path.join(directory, raw_filename[5:])
+            from sage.env import SAGE_SRC
+            try_filenames = [os.path.join(directory, raw_filename.removeprefix('sage/'))
                              for directory in sage.__path__]
+            try_filenames.append(os.path.join(SAGE_SRC, raw_filename))  # meson editable install
         else:
             try_filenames = []
         try_filenames.append(
@@ -1286,6 +1288,15 @@ def sage_getfile(obj):
         sage: sage_getfile(P)                                                           # needs sage.libs.singular
         '...sage/rings/polynomial/multi_polynomial_libsingular...'
 
+    Another bug with editable meson install::
+
+        sage: P.<x,y> = QQ[]
+        sage: I = P * [x,y]
+        sage: path = sage_getfile(I.groebner_basis); path
+        '.../sage/rings/qqbar_decorators.py'
+        sage: path == sage_getfile(sage.rings.qqbar_decorators)
+        True
+
     A problem fixed in :issue:`16309`::
 
         sage: cython(                                                                   # needs sage.misc.cython
@@ -1325,7 +1336,7 @@ def sage_getfile(obj):
         return ''
     for suffix in import_machinery.EXTENSION_SUFFIXES:
         if sourcefile.endswith(suffix):
-            return sourcefile[:-len(suffix)]+os.path.extsep+'pyx'
+            return sourcefile.removesuffix(suffix)+os.path.extsep+'pyx'
     return sourcefile