Interface representing a bidirectional transport.
There may be several implementations, but typically, the user does
not implement new transports; rather, the platform provides some
useful transports that are implemented using the platform's best
practices.
The user never instantiates a transport directly; they call a
utility function, passing it a protocol factory and other
information necessary to create the transport and protocol. (E.g.
EventLoop.create_connection() or EventLoop.create_server().)
The utility function will asynchronously create a transport and a
protocol and hook them up by calling the protocol's
connection_made() method, passing it the transport.
The implementation here raises NotImplemented for every method
except writelines(), which calls write() in a loop.