Remove incorrect platform restriction for zlib.ZLIBNG_VERSION

[StanFromIreland]

IIRC we compile Windows releases with zlib-ng, but there is nothing stopping you from doing so on other platforms. The constant is also not documented as "Availability: Windows only".

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[emmatyping]

Hi Stan! Thank you for bringing this up. Couple of points:

1. Zlib-ng is not available on Unix yet: Use zlib-ng (fast!) rather than mainline stale zlib in binary releases cpython#91349 (comment)
   I suppose you could install zlib-ng as zlib and maybe it would work? But you wouldn't get the right includes to define this variable I don't think.
2. Changes to typeshed should go to https://github.com/python/typeshed. Mypy vendors typeshed but changes should preferably made to typeshed itself, then synced here. I'd recommend opening an issue on the typeshed issue tracker to discuss this.

I have plans to modify the CPython Unix build to allow building with zlib-ng so it would be good to clarify what the availability of it is (feel free to ping me on a typeshed issue)

For now though, I am going to close this PR because there isn't anything mypy-specific to be done here. Hope that's okay!

[StanFromIreland]

I suppose you could install zlib-ng as zlib and maybe it would work? But you wouldn't get the right includes to define this variable I don't think.

It is present in the Fedora builds, and it is compiled for me with zlib-ng present on my system, for example:

$ python3.14
Python 3.14.0 (main, Oct  7 2025, 00:00:00) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import zlib
>>> zlib.ZLIBNG_VERSION
'2.2.5'

As for mypy vs. typeshed, apologies I got mixed up between the two!

[emmatyping]

It is present in the Fedora builds

Good to know!