meson: simplify lookup of python command

It can be treated like any other command, we don't need a full blown
module capable of building extensions just to get an ExternalProgram
executable that can be used to run scripts.

Since find_program has a builtin kwarg for requiring a given version, we
can avoid manually coding some checks and emitting a custom error.
10 jobs for find_program_python3 in 16 minutes and 21 seconds (queued for 2 seconds)