Merge cred-implements-implementer-7620-2: Replace implements with implementer in t.cred.credentials

Author: wolfgang61
Reviewer: jml
Fixes: #7620

The @implementer class decorator is a Python 3-safe way of declaring
that a class implements an interface.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43387 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb