demandimport: add __future__ to ignore list

__future__ is special. We should definitely not be trying to lazy
load it.

mercurial/demandimport.py

@@ -145,6 +145,7 b' def _demandimport(name, globals=None, lo'
         return mod
 
 ignore = [
+    '__future__',
     '_hashlib',
     '_xmlplus',
     'fcntl',