test(sandbox): make local read error regression portable

Author: matthewflint

tests/sandbox/test_entries.py

@@ -281,7 +281,8 @@ async def test_local_file_preserves_local_read_error_when_write_wraps_stream_fai
     source = (tmp_path / "source.txt").resolve()
     source.write_bytes(b"original")
     session = _PayloadWrappingWriteSession()
-    original_open = Path.open
+    source_path_type = type(source)
+    original_open = source_path_type.open
 
     def failing_open(
         self: Path,
@@ -295,7 +296,7 @@ def failing_open(
             return _FailAfterChunkStream(b"original")
         return cast(io.IOBase, original_open(self, mode, buffering, encoding, errors, newline))
 
-    monkeypatch.setattr(Path, "open", failing_open)
+    monkeypatch.setattr(source_path_type, "open", failing_open)
 
     with pytest.raises(LocalFileReadError) as excinfo:
         await LocalFile(src=Path("source.txt")).apply(