scons: add py3 support

SCons 3.1 has moved to python 3, requiring this fix
to continue supporting scons builds.

Closes: #944
Cc: mesa-stable@lists.freedesktop.org
Acked-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Eric Engestrom <eric@engestrom.ch>
(cherry picked from commit 3f92d178)