When writing to a property that doesn't exist e.g. `frame.drawing.strokes.test = 42`
no exception would be raised and it would silently fail.
The fix defines the `__slots__` on the classes explicitly which then raises an exception
if the user tries to write something that wasn't previously defined.
Pull Request: https://projects.blender.org/blender/blender/pulls/129047
251250a3fb