Can't run tests on python 3.11

```
______________________ ERROR collecting tests/rpc_test.py ______________________
tests/rpc_test.py:22: in <module>
    class MyHandler(aiozmq.rpc.AttrHandler):
tests/rpc_test.py:55: in MyHandler
    @asyncio.coroutine
E   AttributeError: module 'asyncio' has no attribute 'coroutine'
```

`asyncio.coroutine` was removed in Python 3.11 but is still used in `tests/rpc_test.py`