> - From the docs, I could not understand the difference between
> sys.implementation.version and sys.version_info.  When can they differ?

I'll make an update.  As an example, PyPy is at version 1.8 which implements the Python 2.7 language.  In that case sys.version_info would be (2, 7, 3, 'final', 0) while sys.implementation.version might be (1, 8, 0, 'final', 0).

One confusing part is that for CPython they are exactly the same.  I think there's room for improvement with the versioning of the language specification itself, but one thing at a time!  :)

> - _PyNamespace_New should be a public API function.  From Python code,
> SimpleNamespace is public.
> - I agree that _PyNamespace_Type could stay private (it's an
> implementation detail), in this case PyAPI_DATA is not necessary.

Agreed, though I'd like to see sys.implementation go in first and take action on the PyNamespace type/API in a separate issue.

> - SimpleNamespace should support weak references.

I'll admit that I haven't used weakrefs a ton and am not familiar with the use cases.  However, the fact that most of the builtin types do not support weak references gives me pause to simply adding it.

I'd like more discussion on this, but in a separate issue so that it doesn't hold up sys.implementation.