[Python][FlightRPC] Some flight classes not exported as public

[vtk9]

Describe the bug, including details regarding any error messages, version, and platform.

For example, _flight.ServerAuthSender is not exposed in pyarrow.flight

So we cannot write

import pyarrow.flight as flight

def authenticate(self, outgoing: flight.ServerAuthSender, incoming: flight.ServerAuthReader):
    pass

without the IDE/linter erroring out that ServerAuthSender is not a known member of module flight

There's also no documentation for it in https://arrow.apache.org/docs/python/generated/pyarrow.flight.ServerAuthHandler.html

Component(s)

FlightRPC, Python