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/common.py

@@ -34,8 +34,8 @@ hostname_regex = re.compile(
 @zope.interface.implementer(interfaces.IPlugin)
 class Plugin(object):
     """Generic plugin."""
-    # classProvides is not inherited, subclasses must define it on their own
-    #zope.interface.classProvides(interfaces.IPluginFactory)
+    # provider is not inherited, subclasses must define it on their own
+    # @zope.interface.provider(interfaces.IPluginFactory)
 
     def __init__(self, config, name):
         self.config = config

letsencrypt/plugins/manual.py

@@ -24,6 +24,7 @@ logger = logging.getLogger(__name__)
 
 
 @zope.interface.implementer(interfaces.IAuthenticator)
+@zope.interface.provider(interfaces.IPluginFactory)
 class Authenticator(common.Plugin):
     """Manual Authenticator.
 
@@ -35,7 +36,6 @@ class Authenticator(common.Plugin):
     .. todo:: Support for `~.challenges.TLSSNI01`.
 
     """
-    zope.interface.classProvides(interfaces.IPluginFactory)
     hidden = True
 
     description = "Manually configure an HTTP server"

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

letsencrypt/plugins/standalone.py

@@ -136,6 +136,7 @@ def supported_challenges_validator(data):
 
 
 @zope.interface.implementer(interfaces.IAuthenticator)
+@zope.interface.provider(interfaces.IPluginFactory)
 class Authenticator(common.Plugin):
     """Standalone Authenticator.
 
@@ -144,7 +145,6 @@ class Authenticator(common.Plugin):
     challenges from the certificate authority. Therefore, it does not
     rely on any existing server program.
     """
-    zope.interface.classProvides(interfaces.IPluginFactory)
 
     description = "Automatically use a temporary webserver"
 

letsencrypt/plugins/webroot.py

@@ -18,9 +18,9 @@ logger = logging.getLogger(__name__)
 
 
 @zope.interface.implementer(interfaces.IAuthenticator)
+@zope.interface.provider(interfaces.IPluginFactory)
 class Authenticator(common.Plugin):
     """Webroot Authenticator."""
-    zope.interface.classProvides(interfaces.IPluginFactory)
 
     description = "Webroot Authenticator"