WebTransport server does not support live-streaming

WebTransport server does not support live-streaming (media streams; data streams).

Steps to reproduce: Modify webtransport_server.py to stream content using, for example

```
for c in iter(lambda: ''.join(choice(digits) for i in range(512)).encode('ascii'), b''): # replace '' with b'' for Python 3
if c is not None:
self._http._quic.send_stream_data(
event.stream_id, c, end_stream=False)
```

and

```
cmd = 'parec', '-d', 'alsa_output.pci-0000_00_1.analog-stereo.monitor'
process = subprocess.Popen(cmd, stdout=subprocess.PIPE)
os.set_blocking(process.stdout.fileno(), False)
for c in iter(lambda: process.stdout.read(512), b''):
if c is not None:
self._http._quic.send_stream_data(
event.stream_id, c, end_stream=False)
```

Expected result: Data to be streamed to `WebTransport` client.
Actual result: No data is streamed to client.
Related: https://github.com/aiortc/aioquic/issues/226.