This PR adds type var `T_Event` and uses it to type the `subscribe()` interfaces.

Fixes an issue where subscribing a handler would raise a type error if the handler is typed to receive a subclass of `Event`.

Allows type checkers to verify that the handler passed to `subscribe()` can support the event types it is assigned to handle.

Closes agronholm#846

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Relaxes the signatures of methods on `MQTTEventBroker` that are registered as callbacks on the mqtt client so that they will work as either v1 or v2 client style callbacks.

Adds a tox env to explicitly test against paho-mqtt v1 (`$ tox -e paho-mqtt-v1`).

This commit keeps the behavior of the subscribe methods as they are for the case where a subscription is added for an iterable of event types. However, for the case where a subscription is added for a single event type, it allows the callable to be typed to receive an instance of that event type, removing the need to type-narrow in that case.

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