A self-contained piece of Michael Hudson's patch

#449043 supporting __future__ in simulated shells
in support of PEP 264.

Much has changed from the patch version:
+ Repaired bad hex constant for nested_scopes.
+ Defined symbolic CO_xxx names so global search will find these uses.
+ Made the exported list of feature names explicit, instead of abusing
  __all__ for this purpose (and redefined __all__ accordingly).
+ Added gross .compiler_flag verification to test___future__.py, and
  reworked it a little to make use of the newly exported explicit list
  of feature names.

Lib/test/test___future__.py

@@ -5,7 +5,7 @@ import __future__
 
 GOOD_SERIALS = ("alpha", "beta", "candidate", "final")
 
-features = [x for x in dir(__future__) if x[:1] != "_"]
+features = __future__.all_feature_names
 for feature in features:
     value = getattr(__future__, feature)
     if verbose:
@@ -39,3 +39,8 @@ for feature in features:
         verify(type(serial) is IntType, "mandatory serial isn't int")
         verify(optional < mandatory,
                "optional not less than mandatory, and mandatory not None")
+
+    verify(hasattr(value, "compiler_flag"),
+           "feature is missing a .compiler_flag attr")
+    verify(type(getattr(value, "compiler_flag")) is IntType,
+           ".compiler_flag isn't int")