Add serial port support

[agronholm]

We should look at pyserial to figure out what needs to be done to support I/O on serial ports. Hopefully C extensions or CFFI are not needed.

[agronholm]

One thing looks clear: Serial port initialization and handling is really tricky. We don't want to duplicate pyserial's efforts, but maybe provide stream classes that work through pyserial.

[smurfix]

https://github.com/M-o-a-T/anyio_serial.git does that. Granted that delegating absolutely everything to a worker thread is not an optimal solution by a long shot, but at least it gets the job done.

[agronholm]

Just a quick observation: your README uses markdown syntax but the file suffix is .rst.

[smurfix]

Thanks, that file did need some attention. :-/

[agronholm]

I have a number of other changes I'd like to make in that repo. Can we talk in the AnyIO Gitter room?

[agronholm]

Windows + asyncio is the only problematic point here. It might be possible to use the private API of the proactor event loop for doing overlapped I/O on the serial port. On other platforms, all we have to do is add a reader for it. I'll take a stab at this for v3.1 if I manage to getmyself a USB-serial converter.

[agronholm]

I found code that can do asynchronous serial I/O on ProactorEventLoop without (user-space) threads: https://github.com/m-labs/asyncserial/blob/master/asyncserial/asyncserial.py