C0103 fixes 3, dht, k_bucket and routing_table

This PR is dependent on #1108.
`_iterativeFind` is meant to be a private method of the `dht` object, as denoted by the leading `_`. Obviously, we haven't been following our own convention, but I don't think the proper way to resolve it is to make `_iterativeFind` public. In this particular case, it would be more appropriate to call
```python
self.dht.iterative_find_node(self.guid)
```