Avoid TraitError when setting root_dir

Author: kevin-bates

pytest_jupyter/jupyter_server.py

@@ -243,7 +243,7 @@ def jp_kernelspecs(jp_data_dir):
 @pytest.fixture(params=[True, False])
 def jp_contents_manager(request, jp_root_dir):
     """Returns a FileContentsManager instance based on the use_atomic_writing parameter value."""
-    return FileContentsManager(root_dir=jp_root_dir, use_atomic_writing=request.param)
+    return FileContentsManager(root_dir=str(jp_root_dir), use_atomic_writing=request.param)
 
 
 @pytest.fixture