--- a/mercurial/templateutil.py	Wed Nov 06 15:15:00 2019 -0500
+++ b/mercurial/templateutil.py	Wed Nov 06 17:46:12 2019 -0500
@@ -111,6 +111,8 @@
 class mappable(object):
     """Object which can be converted to a single template mapping"""
 
+    __metaclass__ = abc.ABCMeta
+
     def itermaps(self, context):
         yield self.tomap(context)