scons,python: Prioritize Python3 for PYTHON_CONFIG

Change-Id: I0ac4d90b93f2e0a9384216f759937f7b0aa23d41
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34899
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed