demandimport: support importing builtins for Python 3

__builtin__ was renamed to builtins in Python 3. If importing
__builtin__ fails, fall back to importing builtins in order to support
Python 3.

mercurial/demandimport.py

@@ -26,7 +26,12 b' These imports will not be delayed:'
 
 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__