find_vpython.sh: POSIX.2 script to find a matching version python.

The script verify if PYTHON is set in the environment (to match what Autotools expect); if not, it searches for all '^python[0-9.]*$' in the PATH. Then, for this list of candidates, it searches for the highest matching version python. The version (optional) is passed as op major [minor [micro]]. Op is one of '<', '<=', '==', '>=' or '>'. It exits with 0 on success and then publish on stdout the pathname of python, a space, and the pythondir (subdir where the third parties modules will be placed).

To be used with Meson (and copied in the module dir).