Class `AbstractRobustChannel` has not `ready(...)` method

# Issue

```python3
# in a corofunc
conn = await connect_robust(amqp_url)
channel = await conn.channel()
```

Here, var `channel` has a type hinting of `AbstractRobustChannel`, and its runtime type is `RobustChannel`.
But, class `AbstractRobustChannel` has not `ready(...)` method.

# Version info
aio-pika                9.5.5