copy _subscriptions so that it can't be altered within the loop

[tillschoepe]

Fix for Dictionary Size Change During Iteration in _on_connect

This PR addresses Issue #20, where _on_connect in _client.py (line 168) fails when multiple clients are running and one client publishes a new topic while another subscribes. The root cause is that self._subscriptions is modified during iteration, leading to a RuntimeError: dictionary changed size during iteration.

Fix

I converted the iterator into a list before iterating over self._subscriptions. This ensures that the loop does not reference the dictionary directly, preventing modifications during iteration.

Changes

def _on_connect(self, rc: int) -> None:
    check_connack_code(rc)
    self._births.clear()
    for topic, qos in list(self._subscriptions.items()):  # Convert to list to avoid runtime error
        self._subscribe(topic=topic, qos=qos)