PyAPI: Use annotations for RNA definitions

- Logical use of fields since they define type information.
- Avoids using ordered-dict metaclass.

Properties using regular assignments will print a warning and load,
however the order is undefined.

release/scripts/startup/bl_operators/image.py

@@ -29,7 +29,7 @@ class EditExternally(Operator):
     bl_label = "Image Edit Externally"
     bl_options = {'REGISTER'}
 
-    filepath = StringProperty(
+    filepath: StringProperty(
         subtype='FILE_PATH',
     )