Merge inlineCallbacks-return-7624: allow 'return' in python 3.3+

Author: Alex

Reviewer: glyph

Fixes: #7624

Add the ability to use 'return' to supply a Deferred result, obviating
the need for returnValue, in versions of Python which syntactically
support this.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43019 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb