Fix naming of ABI3 extension libraries on Windows

[wjakob]

This commit fixes the naming conventions of ABI3 builds on Windows. It also adopts a recent CMake/FindPython interface whenever available.

[henryiii]

I'd recommend basing this on the existence of these variables, rather than the CMake version, because scikit-build-core has 3.26's FindPython even on 3.15 (unless a user disables it).

[henryiii]

Though I guess Python_SOABI was incorrect for PyPy until 3.26 as well. Generally I’d try to avoid calling Python, as it could be incorrect if you are cross compiling, and it's slower than just using a value. Maybe just use these values if they are defined unless it's PyPy & older CMake? (very minor issue)

[wjakob]

I'd recommend basing this on the existence of these variables [..]

ok will do.

Python_SOABI seems not to include the final part of the extension (.dll / .so). Is there a portable way to get that? I am asking because of the inconsistency on macOS, where .so is used instead of .dylib.

[wjakob]

Merging in favor of #221.