on_pre_connect not optional?

Hi,

I just migrated a ( rather large) script to 1.6.1, and for some reason I had to add something like this

```
self.client.on_pre_connect = self.on_pre_connect

def on_pre_connect(self, *args):
"""This seems to be needed due to a bug in paho-1-6-1"""
self.logger.debug('running on_pre_connect, apparently paho needs this now')
```

for it to connect. Otherwise it would complain about the missing overwrite above. After quickly glancing at the code this seems to be meant as optional, which for me it apparently is not.

KR,
Moritz