GH-144679: Upgrade to windows-2025-vs2026 in GitHub Actions

[chris-eibl]

Per offline discussion with @zooba we'd like to switch over to VS2026 on main already now. In May windows-2025 will get VS2026, anyway.

Yeah, it's released and stable, so let's switch main over

Just the same changes that @vstinner did in #144682 for windows-2025.

• Issue: When building with VS2026 on Windows, use PlatformToolSet v145 by default #144679

[savannahostrowski]

Oh great! Thanks for doing this!

[StanFromIreland]

The Windows CI won't run till #144674 is merged.

[vstinner]

The Windows CI won't run till #144674 is merged.

As a workaround, you can push a test change modifying a file in another directory, check the CI, and then revert this test change.

[hugovk]

#144674 is now merged and I merged in main, so we can remove the dummy commit.

[zooba]

v1950 is the compiler version you expect to see in sys.copyright after this change? (The builds don't seem to be using the -v flag, which is why you can't see the compiler commands as they build)

[chris-eibl]

Yeah, I've verified. You see it in the build logs when the tests start.

Edit: or in "Display build info"

sys.version: 3.15.0a6+ (remotes/pull/145005/merge:ee4fd43, Feb 19 2026, 17:02:53) [MSC v.1950 64 bit (AMD64)]

[chris-eibl]

E.g. https://github.com/python/cpython/actions/runs/22191597199/job/64180695117?pr=145005

== CPython 3.15.0a6+ (remotes/pull/145005/merge:ee4fd43, Feb 19 2026, 17:02:53) [MSC v.1950 64 bit (AMD64)]