Steps to Reproduce

1. Check slots of _BaseMedium.
2. accessing bot in any instance of _BaseMedium would lead to an error since that is not used. We use _bot.

Expected behaviour

bot should be removed from the slot.

Actual behaviour

Operating System

Version of Python, python-telegram-bot & dependencies

PTB v14

Relevant log output

No response

Additional Context

test_slot_behaviour passes for PhotoSize since we don't explicitly have a test for its superclass (_BaseMedium).

Will be fixed in #2893