Fix Python 3 compatibility issues.

Fix two issues with Python 3:
* Use class annotations to denote an implementation of a
  Zope interface;
* A classical byte/str issue.

incenp/certbot/ssh.py

@@ -13,6 +13,8 @@ from subprocess import Popen, PIPE
 logger = logging.getLogger(__name__)
 
 
+@zope.interface.implementer(interfaces.IAuthenticator)
+@zope.interface.provider(interfaces.IPluginFactory)
 class Authenticator(common.Plugin):
     """SSH Authenticator.
 
@@ -22,8 +24,6 @@ class Authenticator(common.Plugin):
     HTTP.
 
     """
-    zope.interface.implements(interfaces.IAuthenticator)
-    zope.interface.classProvides(interfaces.IPluginFactory)
 
     description = "Send authentication tokens through SSH"
 
@@ -58,7 +58,7 @@ class Authenticator(common.Plugin):
 
         ssh = Popen(["ssh", self.conf("server"), self.conf("remote-command")],
                     stdin=PIPE)
-        ssh.stdin.write(lines)
+        ssh.stdin.write(lines.encode('UTF-8'))
         ssh.stdin.close()
         ssh.wait()