SF patch #1438387, PEP 328: relative and absolute imports.

- IMPORT_NAME takes an extra argument from the stack: the relativeness of
   the import. Only passed to __import__ when it's not -1.

 - __import__() takes an optional 5th argument for the same thing; it
   __defaults to -1 (old semantics: try relative, then absolute)

 - 'from . import name' imports name (be it module or regular attribute)
   from the current module's *package*. Likewise, 'from .module import name'
   will import name from a sibling to the current module.

 - Importing from outside a package is not allowed; 'from . import sys' in a
   toplevel module will not work, nor will 'from .. import sys' in a
   (single-level) package.

 - 'from __future__ import absolute_import' will turn on the new semantics
   for import and from-import: imports will be absolute, except for
   from-import with dots.

Includes tests for regular imports and importhooks, parser changes and a
NEWS item, but no compiler-package changes or documentation changes.

Lib/__future__.py

@@ -51,6 +51,7 @@ all_feature_names = [
     "nested_scopes",
     "generators",
     "division",
+    "absolute_import",
 ]
 
 __all__ = ["all_feature_names"] + all_feature_names
@@ -62,6 +63,7 @@ __all__ = ["all_feature_names"] + all_feature_names
 CO_NESTED            = 0x0010   # nested_scopes
 CO_GENERATOR_ALLOWED = 0        # generators (obsolete, was 0x1000)
 CO_FUTURE_DIVISION   = 0x2000   # division
+CO_FUTURE_ABSIMPORT  = 0x4000   # absolute_import
 
 class _Feature:
     def __init__(self, optionalRelease, mandatoryRelease, compiler_flag):
@@ -102,3 +104,7 @@ generators = _Feature((2, 2, 0, "alpha", 1),
 division = _Feature((2, 2, 0, "alpha", 2),
                     (3, 0, 0, "alpha", 0),
                     CO_FUTURE_DIVISION)
+
+absolute_import = _Feature((2, 5, 0, "alpha", 1),
+                           (2, 7, 0, "alpha", 0),
+                           CO_FUTURE_ABSIMPORT)