Fix paho-mqtt typing errors

[JonathanPlasse]

Hi,
I replaced TypeVar with Union because an attribute should not have a type unbound.
And fixed minor type inconsistencies.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

core (https://github.com/home-assistant/core)
- homeassistant/components/mqtt/client.py:376: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, Any, int], None]", variable has type "Optional[_OnConnect]")  [assignment]
- homeassistant/components/mqtt/client.py:377: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, int], None]", variable has type "_OnDisconnect")  [assignment]
- homeassistant/components/mqtt/client.py:380: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, Any, Any], None]", variable has type "Optional[_OnSubscribe]")  [assignment]
- homeassistant/components/mqtt/client.py:381: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, Any, Any], None]", variable has type "Optional[_OnUnsubscribe]")  [assignment]
- homeassistant/components/mqtt/config_flow.py:430: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, Any, Any], Any]", variable has type "Optional[_OnConnect]")  [assignment]

[JonathanPlasse]

I finished fixing the typing errors.

[JonathanPlasse]

Is it normal that the new version has not been published on PyPI?

[srittau]

Yes, the stubs are updated a few times a day. It might take a few hours.