Use @AbstractMethod instead of @abstractproperty

* The latter it's actually deprecated in favor of using @AbstractMethod
  in conjunction with @Property and @example.setter.

Change-Id: I01bf3c91c162ae666a382a160010025ce7aa8778

cumin/transports/__init__.py

@@ -4,7 +4,7 @@
 import shlex
 import sys
 
-from abc import ABCMeta, abstractmethod, abstractproperty
+from abc import ABCMeta, abstractmethod
 
 from ClusterShell.NodeSet import NodeSet
 from tqdm import tqdm
@@ -610,8 +610,8 @@ def commands(self, value):
 
         self._commands = commands
 
-    @abstractproperty
     @property
+    @abstractmethod
     def handler(self):
         """Get and set the `handler` for the current execution.
 
@@ -626,8 +626,8 @@ def handler(self):
             * an event handler class object (not instance)
         """
 
-    @abstractproperty
     @handler.setter
+    @abstractmethod
     def handler(self, value):  # pylint: disable=property-with-parameters; https://github.com/PyCQA/pylint/issues/3600
         """Setter for the `handler` property. The relative documentation is in the getter."""