The `python` command's default behaviour can vary between OSes and even
different versions of the same OS. On some systems, `python` might still
point to Python 2 for backward compatibility, while on others, it might
point to Python 3.
As Mesa already requires "Python 3.6 or newer", use `python3` explicitly
to ensure that the script is using a Python 3 interpreter.
Moreover, this commit allows this script to run on macOS, as macOS doesn't
have a `python` symlink or alias by default. Therefore, currently, when
running this script in macOS, you get a "python: command not found" error.
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35918>
