Fix python 3.9 compatibility

Python 3.9 renamed an attribute on a threading object, try
both names to support old and new python

confluent_server/confluent/config/configmanager.py

@@ -2548,8 +2548,12 @@ class ConfigManager(object):
         if statelessmode:
             return
         with cls._syncstate:
+            try:
+                isalive = cls._cfgwriter.isAlive()
+            except AttributeError:
+                isalive = cls._cfgwriter.is_alive()
             if (cls._syncrunning and cls._cfgwriter is not None and
-                    cls._cfgwriter.isAlive()):
+                    isalive):
                 cls._writepending = True
                 return
             if cls._syncrunning:  # This suggests an unclean write attempt,