mercurial/demandimport.py

--- a/mercurial/demandimport.py	Sat Jun 27 17:29:15 2015 -0700
+++ b/mercurial/demandimport.py	Sat Jun 27 17:31:06 2015 -0700
@@ -26,7 +26,12 @@
 
 import os, sys
 from contextlib import contextmanager
-import __builtin__ as builtins
+
+# __builtin__ in Python 2, builtins in Python 3.
+try:
+    import __builtin__ as builtins
+except ImportError:
+    import builtins
 
 _origimport = __import__