fix(ci): pin Python discovery to the intended interpreter

[henryiii]

🤖 AI text below 🤖

Description

CI has been red since the manylinux/musllinux images added Python 3.15 and dropped 3.13t, and the same runner refresh broke the free-threaded jobs on macOS and Windows:

• In the manylinux jobs, uv creates the requested venv, but Python_ROOT_DIR is only a hint: FindPython's version scan tries 3.15 first and finds /usr/local/bin/python3.15 on PATH before considering the venv's older interpreter. All four jobs then built against 3.15 and failed at the pytest step (No module named pytest).
• In the standard macOS/Windows 3.14t jobs, FindPython picked a system GIL 3.14 (e.g. Homebrew's framework build) instead of setup-python's free-threaded interpreter.

Fixes:

• CMakeLists.txt: when the venv machinery sets Python_ROOT_DIR, also pin Python_EXECUTABLE to the venv's interpreter, making discovery exact instead of heuristic.
• reusable-standard.yml: pass setup-python's interpreter to CMake as -DPYTHON_EXECUTABLE (works in both old and new FindPython modes).
• ci.yml: drop the 3.13t manylinux rows; the images no longer ship 3.13t, and 3.14t rows already exist.

Suggested changelog entry:

[rwgk]

I just triggered a rerun of the failed

🐍 (ubuntu-latest, 3.13, -DCMAKE_CXX_STANDARD=23 -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON) / 🧪

job, to see if it is a flake.

EDIT: The rerun succeeded:

🐍 (ubuntu-latest, 3.13, -DCMAKE_CXX_STANDARD=23 -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON) / 🧪

[rwgk]

Suggested revised title:

fix(ci): use the intended Python interpreter for CMake discovery

All suggestions aim to avoid using the term "pin", it doesn't seem quite fitting.

Thanks for taking care of this!

[rwgk]

Suggested change

	# FindPython's version scan can pick a different interpreter (e.g.
	# Homebrew's 3.14 instead of setup-python's 3.14t), so pin it.
	# Pass setup-python's resolved interpreter path to CMake so FindPython
	# does not select a different interpreter (e.g. Homebrew's 3.14 instead
	# of setup-python's 3.14t).

[rwgk]

Suggested change

	# Python_ROOT_DIR is only a hint; a newer Python elsewhere on the system
	# still wins the version scan, so pin the venv's interpreter directly.
	# Python_ROOT_DIR is only a search hint, so FindPython may select a newer
	# Python elsewhere. Set Python_EXECUTABLE to the venv's interpreter path
	# to bypass interpreter discovery.