Handle paho-mqtt v1/v2 compat.

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`).

pyproject.toml

@@ -54,12 +54,12 @@ test = [
     "anyio[trio]",
     "asyncmy >= 0.2.5; python_implementation == 'CPython'",
     "coverage >= 7",
-    "paho-mqtt >= 2.0",
+    "paho-mqtt >= 1.5",
     "psycopg",
     "pymongo >= 4",
     "pymysql[rsa]",
     "PySide6 >= 6.6; python_implementation == 'CPython'",
-    "pytest >= 8.0",
+    "pytest >= 7.4",
     "pytest-lazy-fixtures",
     "pytest-mock",
     "time-machine >= 2.13.0; python_implementation == 'CPython'",
@@ -132,4 +132,7 @@ commands = pyright --verifytypes apscheduler
 [testenv:docs]
 extras = doc
 commands = sphinx-build -n docs build/sphinx
+
+[testenv:paho-mqtt-v1]
+deps = paho-mqtt~=1.5
 """