This allows Python scripts to easily determine what Slot a Channelbag is
for. This is particularly important because we're trying to discourage
the use of Slot handles in the Python APIs, and before this the only
way to identify which Slot a Channelbag was for was via the Channelbag's
`slot_handle` property.
Pull Request: https://projects.blender.org/blender/blender/pulls/134053
33b9d54abb