issues/236 -- address DeprecationWarning in `click`

In [`click==8.0.0`](https://click.palletsprojects.com/en/8.0.x/changes/#version-8-0-0), `MultiCommand.resultcallback` is renamed to `result_callback`, along with a deprecation warning. This change adopts the new naming convention.

piku.py

@@ -1110,7 +1110,7 @@ def piku():
     pass
 
 
-@piku.resultcallback()
+@piku.result_callback()
 def cleanup(ctx):
     """Callback from command execution -- add debugging to taste"""
     pass