CMake: react to python version changes

[wjakob]

Description

The new FindPython-based variant of the CMake scripts caches information
about the chosen Python version that can become stale. For example,
suppose I configure a simple pybind11-based project as follows

cmake -S . -B build -GNinja -DPython_ROOT=<path to python 3.8>

which will generate my_extension.cpython-38-x86_64-linux-gnu.so.
A subsequent change to the python version like

cmake -S . -B build -GNinja -DPython_ROOT=<path to python 3.9>

does not update all necessary build system information. In particular,
the compiled file is still called
my_extension.cpython-38-x86_64-linux-gnu.so.

This commit fixes the problem by detecting changes in
Python_EXECUTABLE and re-running Python as needed.

Note that the previous way of detecting Python does not seem to be
affected, it always specifies the right suffix.

Suggested changelog entry:

* Cached Python version information could become stale when CMake was re-run with a different Python version. The build system now detects this and updates this information.

[wjakob]

Amazing! (the pre-commit fixes :D)

[henryiii]

Suggested change

	if(NOT ${_Python}_EXECUTABLE STREQUAL ${_Python}_EXECUTABLE_LAST)
	# Detect changes to the Python version/binary in subsequent CMake runs, and refresh config if needed
	unset(PYTHON_IS_DEBUG CACHE)
	unset(PYTHON_MODULE_EXTENSION CACHE)
	set(${_Python}_EXECUTABLE_LAST
	if(NOT ${_Python}_EXECUTABLE STREQUAL PYTHON_EXECUTABLE_LAST)
	# Detect changes to the Python version/binary in subsequent CMake runs, and refresh config if needed
	unset(PYTHON_IS_DEBUG CACHE)
	unset(PYTHON_MODULE_EXTENSION CACHE)
	set(PYTHON_EXECUTABLE_LAST

If you change from Python2 to Python3, then this should still be regenerated, since these variables are not tied to ${_Python}. (_PYTHON_EXECUTABLE_LAST would also be fine as a name)

[Skylion007]

Agree with @henryiii, otherwise this looks good.

[wjakob]

(Thanks for the feedback, I plan to merge this once CI passes.)

[wjakob]

Thanks, I wondered how this actually managed to pass my (local) testing. Now it makes sense, I'm again removing the FORCE flag.

[skoslowski]

This is great.

May I suggest to use a prefixed name, e.g. PYBIND11_PYTHON_EXECUTABLE_LAST, instead? This makes it easier to identify in bigger trees and avoids collisions.