Implement `get_protocol` and `set_protocol`

[puddly]

These methods are not implemented by BaseProtocol: https://docs.python.org/3/library/asyncio-protocol.html#asyncio.BaseTransport.set_protocol. Not sure if more needs to be done to support protocol switching but this is all my use case requires.

As an aside, is pyserial-asyncio still maintained? There are quite a few outstanding PRs and while more than a couple have been merged, no release has been made since 2021.