Change zope's classProvides to be a class decorator.

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

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

Following the listed advice seems to function in Python 3.

letsencrypt/plugins/null.py

@@ -12,9 +12,9 @@ logger = logging.getLogger(__name__)
 
 
 @zope.interface.implementer(interfaces.IInstaller)
+@zope.interface.provider(interfaces.IPluginFactory)
 class Installer(common.Plugin):
     """Null installer."""
-    zope.interface.classProvides(interfaces.IPluginFactory)
 
     description = "Null Installer"
     hidden = True