This adds two new python properties to `GreasePencilStrokePoint` * `select_handle_left`: The selection of the left handle of this point * `select_handle_right`: The selection of the right handle of this point These could already be written to using `drawing.attributes[".selection_handle_left"]` and `drawing.attributes[".selection_handle_right"]`, but this makes it easier to work on individual points from the higher-level python API. Resolves #137639. Pull Request: https://projects.blender.org/blender/blender/pulls/137730