feat(cmake): FindPython by default

[henryiii]

Description

This makes FindPython the default, and introduces a COMPAT mode that sets common variables to reduce breakage. FindPythonInterp/Libs was removed in CMake 3.27, and has a lot of downsides like not supporting cross compilation (like for Pyodide) properly.

Suggested changelog entry:

* Enable FindPython mode by default, with a ``COMPAT`` mode that sets some of the old variables to ease transition.

[rwgk]

LGTM — based only on very superficial understanding

[PhilipDeegan]

I may be mistaken, but it seems as a result of this, if we find python ourselves before finding pybind

the target pybind11::embed no longer forwards libpython leading to link errors

[henryiii]

This is the only change recently to CMake that I know of, so it probably is the culprit, but I don't see how yet, it shouldn't have changed anything if you find Python yourself first.

[henryiii]

AH, no, I think I see the problem. if(THING) is false in CMake unless THING is a variable. That means my logic is wrong in this case. Can fix.