scons,python: Add warning for when python3-config is not used

scons,python: Add warning for when python3-config is not used

We cannot say for certain whether 'python-config' is python2 or python3,
but this patch will produce a warning if 'python3-config' is not used,
stating that support for python2 will be dropped in future releases of
gem5.

Change-Id: I114da359c8768071bf7dd7f2701aae85e3459678
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35256
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>