This allows detecting if deleting the attribute will be possible
without actually trying to delete it, which can give warning
messages in the terminal.
As an example:
```
>>> [(item.name, item.is_required) for item in mesh.attributes]
[('Attribute', False),
('position', True),
('.edge_verts', True),
('sharp_face', False),
('.corner_vert', True),
('.corner_edge', True)]
```
Pull Request: https://projects.blender.org/blender/blender/pulls/111468
99f9a53321