Event: add dummy _reset_internal_locks
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
add greentest/2.7pypy/
rename _semaphore.pyx -> _semaphore.py

on PyPy we do not build it; on CPython .so takes priority over .py.
WIP pypy support

based on gevent-on-pypy/pypycore@f04a41c written by

Ralf Schmitt
Lucas Clemente Vella
@hasenj
Armin Rigo