Use hardcoded list of CallbackAPIVersions

src/paho/mqtt/client.py

@@ -772,7 +772,7 @@ def __init__(
             raise ValueError(
                 "Unsupported callback API version: version 2.0 added a callback_api_version, see docs/migrations.rst for details"
             )
-        if self._callback_api_version not in CallbackAPIVersion:
+        if self._callback_api_version not in [CallbackAPIVersion.VERSION1, CallbackAPIVersion.VERSION2]:
             raise ValueError("Unsupported callback API version")
 
         self._clean_start: int = MQTT_CLEAN_START_FIRST_ONLY

src/paho/mqtt/enums.py

@@ -1,7 +1,7 @@
 try:
     import enum
 except ImportError:
-    from . import simpler_enum as enum
+    import .simpler_enum as enum
 
 class MQTTErrorCode(enum.IntEnum):
     MQTT_ERR_AGAIN = -1

src/paho/mqtt/simpler_enum.py

@@ -1,10 +1,11 @@
-
-class IntEnum:
+class SimplerEnum:
     pass
 
-class Enum:
+class Enum(SimplerEnum):
     pass
 
+IntEnum = Enum
+
 class _Auto:
     def __init__(self):
         self._i = -1