Python 3.7 syntax error: async is now a reserved keyword

[aalex]

async is now a reserved keyword in Python 3.7 and up.

Steps to reproduce

Try the example in examples/async_udp_sender.py

Expected behavior

Module should not be named async.

Actual behavior

Module is called async.

    from txosc.async import DatagramClientProtocol
                   ^
SyntaxError: invalid syntax

Technical details

Let's rename that module to asynchronous.