meson: Search for python3 before python for bin/meson_get_version.py

meson: Search for python3 before python for bin/meson_get_version.py

Most systems have either dropped the python executable, or made it
python3.

But it is still possible to configure a system such that python runs
python2. https://www.python.org/dev/peps/pep-0394/

Or, some developers may still be running older distributions where
python is python2.

Since bin/meson_get_version.py now requires python3, we should search
for python3 before python.

Fixes: f1eae2f8bbe ("python: drop python2 support")
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12400>