plugin.py: Redirect DEBUG from `__del__` to `sys.stderr`

Previously, debug messages from `Plugin.__del__()` went to stdout
which was a problem for the test suite in extremely rare and hard to
predict circumstances.  This corrects that by sending the message to
stderr where all the rest of the messages tend to go.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>