Help isinstance() recognize _SelectorDatagramTransport as subtype of asyncio.DatagramProtocol #8051

Consider a custom datagram-only asyncio protocol:

import asyncio

class MyProtocol(asyncio.DatagramProtocol):
    def connection_made(self, transport: asyncio.BaseTransport):  # signature as per typeshed super class
        if not isinstance(transport, asyncio.DatagramTransport):
            raise TypeError("%s not supported because it's not a datagram transport." % type(transport))
        # ...

For UDP connections on Linux, the above code results in

TypeError: <class 'asyncio.selector_events._SelectorDatagramTransport'> not supported because it's not a datagram transport.

Per implementation, class _SelectorDatagramTransport is a structural subtype of class DatagramTransport, but it does not actually inherit. However, from a users perspective I'd say both static and runtime type checks should succeed.

I'm unsure if the required change would have to go to cpython (let _SelectorDatagramTransport inherit DatagramTransport) or to typeshed (define BaseTransport and DatagramTransport as typing.Protocol with @runtime_checkable). What would you say?

If you think this is a valid request, I could try a PR.