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: f1eae2f8 ("python: drop python2 support")