meson: Use a more idiomatic way to find Python 3

We were using the generic find_program function to find the python3
executable.

However, Meson provides a  builtin python3 module which, among other
things, allows doing just that.

http://mesonbuild.com/Python-3-module.html

https://bugzilla.gnome.org/show_bug.cgi?id=781944