list of known failures as a python script

replaced known_failures.txt with known_failures.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 and Lucas Clemente Vella

with patches from hasenj and Armin Rigo