Move __pycache__ directory removal from debian/rules to the clean
Makefile target. The change should have no effect on the generated
packages; it just makes make clean more complete and debian/rules
simpler.
Instead of using a shell loop for passing the files to rm, also use
find's -delete option, avoiding any potential issues with weird
filenames.
Since the __pycache__ folders may contain .pyc files, make sure these
are all deleted before.