Remove zlib.ZLIBNG_VERSION Windows requirement

[StanFromIreland]

$ 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'

[StanFromIreland]

cc @emmatyping

[srittau]

This seems not to be always true:

>>> import zlib; zlib.ZLIBNG_VERSION
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    import zlib; zlib.ZLIBNG_VERSION
                 ^^^^^^^^^^^^^^^^^^^
AttributeError: module 'zlib' has no attribute 'ZLIBNG_VERSION'. Did you mean: 'ZLIB_VERSION'?

(Ubuntu 24.04.3 LTS, Python installed via pyenv.) That said, even if it's only sometimes available, we should add it with an appropriate comment.

[StanFromIreland]

This seems not to be always true:

It varies on how it was built, in the Fedora releases it is present.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Thanks!