Compositor: Remove scene.use_nodes from Python API

Use Nodes were removed in d88d4cc8 from the UI, but marked deprecated
in Python API. Since we decided to remove `world.use_nodes` and
`material.use_nodes` for the shader editor (instead of deprecating
them), we also remove `scene.use_nodes`.

This PR only replaces the deprecated `scene.node_tree` with
`scene.compositing_node_group` where necessary. #143619 will fully
remove `node_tree`

Pull Request: https://projects.blender.org/blender/blender/pulls/143578

tests/python/bl_node_group_interface.py

@@ -430,8 +430,8 @@ class CompositorNodeGroupInterfaceTest(AbstractNodeGroupInterfaceTest, NodeGroup
     def setUp(self):
         super().setUp()
         self.scene = bpy.data.scenes.new("test")
-        self.scene.use_nodes = True
-        self.main_tree = self.scene.node_tree
+        self.main_tree = bpy.data.node_groups.new("test node tree", "CompositorNodeTree")
+        self.scene.compositing_node_group = self.main_tree
 
     def test_invalid_socket_type(self):
         self.do_test_invalid_socket_type("INVALID_SOCKET_TYPE_11!1")

tests/python/bl_pyapi_idprop_datablock.py

@@ -131,8 +131,10 @@ def make_lib():
         bpy.context.collection.objects.link(ob)
 
     # nodes
-    bpy.data.scenes["Scene"].use_nodes = True
-    sys_idprops = bpy.data.scenes["Scene"].node_tree.nodes['Render Layers'].bl_system_properties_get(do_create=True)
+    tree = bpy.data.node_groups.new("Compositor Nodes", "CompositorNodeTree")
+    bpy.data.scenes["Scene"].compositing_node_group = tree
+    rlayers = tree.nodes.new(type="CompositorNodeRLayers")
+    sys_idprops = rlayers.bl_system_properties_get(do_create=True)
     sys_idprops["prop"] = bpy.data.objects['Camera']
 
     # rename scene and save