New Features

ECM-1240 API support
- get_serial_number
- set_packet_send_interval
- ApiCall can be used to add support for more APIs outside of this library
Ability to make GEM API calls without pausing packet sending, making them vastly faster than before. This is experimental for now; pass send_packet_delay=True to BidirectionalProtocol's constructor to enable.
Support for API calls that have no response (pass None for the parser parameters to ApiCall)

Fixes

PacketProtocol no longer has a close() method. Users should close the transports directly. For example, if self._server is an asyncio.Server:

            for s in self._server.sockets:
                s.shutdown(socket.SHUT_RDWR)
                s.close()

BidirectionalProtocol no longer has send_api_request and receive_api_response methods. Instead, there is now an invoke_api method that takes a Future which will be completed as soon as the API response comes in. See README.md for more details on its use. This change allows API responses to be returned to the caller much faster.
The serial_number field for ECM-1240 packets is now treated as a 16-bit little-endian value. Previous versions treated it as a big-endian value. ECM-1240 packets received with this version of the library will thus have a serial number field that is in the opposite byte order from those returned by the previous verson.