The changelog is sloppily worded. from __future__ import absolute_import does not care about whether something is part of the standard library, and import string will not always give you the standard-library module with absolute imports on.
from __future__ import absolute_import means that if you import string, Python will always look for a top-level string module, rather than current_package.string. However, it does not affect the logic Python uses to decide what file is the string module. When you do
python pkg/script.py
pkg/script.py doesn't look like part of a package to Python. Following the normal procedures, the pkg directory is added to the path, and all .py files in the pkg directory look like top-level modules. import string finds pkg/string.py not because it's doing a relative import, but because pkg/string.py appears to be the top-level module string. The fact that this isn't the standard-library string module doesn't come up.
To run the file as part of the pkg package, you could do
python -m pkg.script
In this case, the pkg directory will not be added to the path. However, the current directory will be added to the path.
You can also add some boilerplate to pkg/script.py to make Python treat it as part of the pkg package even when run as a file:
if __name__ == '__main__' and __package__ is None:
__package__ = 'pkg'
However, this won't affect sys.path. You'll need some additional handling to remove the pkg directory from the path, and if pkg's parent directory isn't on the path, you'll need to stick that on the path too.