Revert r37840: Merge child-processes-endpoint-4696-5.

Reopens: #4696

There is a failure on the python3 buildbot.
twisted.internet.test.test_endpoint imports StandardErrorBehavior
unconditionally, although it isn't supported on python3.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@37849 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb