fix: support sysconfig direclty in 3.10+

[henryiii]

Description

Closes #3677.

Suggested changelog entry:

* Use sysconfig over distutils for Python 3.10 using classic CMake discovery.

[rwgk]

This seems to be very similar to a recent change in tools/pybind11NewTools.cmake. Evidently, someone works on one copy, unaware of the other, and it takes two iterations. I usually flag such situations with a comment, e.g.

# Note that there is similar code in X.

[henryiii]

This is a lot more complicated because we'll have to work out how to get each value, and have some sort of nicer toggle to keep the code from exploding. (notice it doesn't work on Python 3.10 with the current change) The pybind11NewTools.cmake change is only partially related.

[henryiii]

(pytorch/pytorch@4b96fc0 will likely be very helpful)

Given that nothing is broken, I think we are fine to continue with a release without doing this.

[stefanor]

Filed PR #3764, which adds a sysconfig path for >= 3.10, and uses the distutils mechanism for older pythons.