Fix cannot import name 'cli' from '__main__' error

  * Closes #69

Author: tcaddy

datadog_callback.py

@@ -16,7 +16,10 @@
 
 import ansible
 from ansible.plugins.callback import CallbackBase
-from __main__ import cli
+try:
+    from __main__ import cli
+except ImportError:
+    cli = True
 
 ANSIBLE_ABOVE_28 = False
 if IMPORT_ERROR is None and version.parse(ansible.__version__) >= version.parse('2.8.0'):