Mark the "with_statement" __future__ feature as no-op since it's always on.

Cython/Compiler/Future.py

@@ -4,7 +4,7 @@ def _get_feature(name):
     return getattr(__future__, name, object())
 
 unicode_literals = _get_feature("unicode_literals")
-with_statement = _get_feature("with_statement")
+with_statement = _get_feature("with_statement")  # dummy
 division = _get_feature("division")
 print_function = _get_feature("print_function")
 absolute_import = _get_feature("absolute_import")