Change zope's implements to be a class decorator.

When attempting to import any module that uses zope.interface.implements
in Python 3, a TypeError is raised; it reads:

    TypeError: Class advice impossible in Python3.  Use the @implementer
    class decorator instead.

Following the listed advice seems to function in Python 3.

letsencrypt/plugins/null.py

@@ -11,9 +11,9 @@ from letsencrypt.plugins import common
 logger = logging.getLogger(__name__)
 
 
+@zope.interface.implementer(interfaces.IInstaller)
 class Installer(common.Plugin):
     """Null installer."""
-    zope.interface.implements(interfaces.IInstaller)
     zope.interface.classProvides(interfaces.IPluginFactory)
 
     description = "Null Installer"