issue #477: fix source of become_flags on 2.3.

ansible_mitogen/transport_config.py

@@ -352,7 +352,11 @@ class PlayContextSpec(Spec):
             mitogen.core.to_text(term)
             for s in (
                 self._play_context.sudo_flags,
-                self._play_context.become_flags
+                self._play_context.become_flags,
+                # Ansible 2.3.
+                getattr(C, 'DEFAULT_BECOME_FLAGS', ''),
+                getattr(C, 'DEFAULT_SUDO_FLAGS', ''),
+
             )
             for term in ansible.utils.shlex.shlex_split(s or '')
         ]