extensions: don't give AttributeError bytes message on Python 3

Differential Revision: https://phab.mercurial-scm.org/D353

mercurial/extensions.py

@@ -396,8 +396,8 b' def wrapfilecache(cls, propname, wrapper'
             break
 
     if currcls is object:
-        raise AttributeError(
-            _("type '%s' has no property '%s'") % (cls, propname))
+        raise AttributeError(r"type '%s' has no property '%s'" % (
+            cls, propname))
 
 def wrapfunction(container, funcname, wrapper):
     '''Wrap the function named funcname in container