**How to reproduce:**
1. Assign a node tree to the scene compositing node group in Python
2. Notice how the node tree has the wrong user count
Example:
```python
C.scene.compositing_node_group = \
D.node_groups.new("ntree", "CompositorNodeTree")
print(C.scene.compositing_node_group.users) # returns 0
```
Pull Request: https://projects.blender.org/blender/blender/pulls/143577
