build: Simplify python discovery at configure time

[dridi]

If the desired python command is newer than the ones known by aclocal, we can use the PYTHON variable to select it:

./configure PYTHON=python3.11

This already worked before this change, but now we stop chasing python releases to get them automatically tested.

Refs #3982

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Keeping as a draft for the moment, considering to hold it until we know the next major version (7.5.0 or 8.0.0) to make sure we don't forget to update the serial.

[catap]

❤️