Merge internet-utils-mergs-6240-2: Merge `twisted/internet/_utilspy3.py` into `twisted/internet/utils.py`.

Author: genericnick, tom.prince
Reviewers: tom.prince, therve
Fixes: #6240

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@37876 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb