scons: Added 'python3-config' to the config list

scons: Added 'python3-config' to the config list

Previously scons looked for 'python-config' and 'python2-config'. This
caused an error in Python3-exclusive environments. With only python3
installed, 'python3-config' is now an option. With this setup,
'python2-config' is used by default, unless overridden by the user
via the scons command line.

Change-Id: I11186328b0eb0825cc53c3889b31a5345a7f40d1
Issue-on: https://gem5.atlassian.net/browse/GEM5-275
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28648
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>