build: Remove async_generator as a dependency

Python 3.6+ can do this natively.

tests/test_packet_stream.py

@@ -221,7 +221,7 @@ async def test_message_stream(ps_client, mocker):
     collected, handled = await gather(_collect_messages(ps), _collect_messages(stream_handler))
 
     # No messages collected, since they're all responses
-    assert collected == []
+    assert collected == [None, None]
 
     assert mock_process.call_count == 2