This broke test_socketserver:

http://buildbot.python.org/all/builders/AMD64%20Windows7%20SP1%203.x/builds/7709/steps/test/logs/stdio
0:08:57 [193/400/1] test_socketserver failed
test test_socketserver crashed -- Traceback (most recent call last):
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\test_socketserver.py", line 371, in <module>
    class ForkingErrorTestServer(socketserver.ForkingMixIn, BaseErrorTestServer):
AttributeError: module 'socketserver' has no attribute 'ForkingMixIn'

I am with David that it may not be worthwhile churning the API like this. Just document the existing (in 3.5) status that even though the classes exist, they need a working os.fork(). Otherwise, everyone that references ForkingMixIn but doesn’t use it (like test_socketserver) will have to update their code for 3.6.