test__socket_dns.py: do not compare IPv6 addresses
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
bigger delays in tests for travis/pypy