Fix Python 3.12 compatibility (#6965)

.joinpath with an empty argument no longer works as before thanks to this change in CPython: python/cpython@cea910e#diff-2e741d925220d74a9cc04cda1314d3649d9d189c0efc7db18e5387a51225b61c

This change mimics the old internal behavior by using _paths attribute directly.

tests/conftest.py

@@ -91,7 +91,7 @@ def _make_notebook_app(**kwargs):
     )
 
     # Copy the schema files.
-    test_data = str(files("jupyterlab_server.test_data").joinpath(""))
+    test_data = str(files("jupyterlab_server.test_data")._paths[0])  # type: ignore
     src = pathlib.PurePath(test_data, "schemas", "@jupyterlab")
     dst = pathlib.PurePath(str(schemas_dir), "@jupyterlab")
     if os.path.exists(dst):