util.py: remove unused variable SLEEP
util.py: ignore decoding errors
threadpool.py: rename internal var

'result' into 'thread_result'
simpler implementation of backdoor.py
WIP pypy support

based on https://github.com/gevent-on-pypy/pypycore/commit/f04a41c0eacfab2662e24291cd63859ecea2f9fe written by

Ralf Schmitt
Lucas Clemente Vella
@hasenj
Armin Rigo