RHEL9 Support - Test cases to leverage unittest.mock when possible (#658)

test/test_plugin_slack.py

@@ -25,7 +25,14 @@
 
 import os
 import six
-import mock
+try:
+    # Python 3.x
+    from unittest import mock
+
+except ImportError:
+    # Python 2.7
+    import mock
+
 import pytest
 import requests
 from apprise import plugins