scons,python: Fix `--without-python` flag

scons,python: Fix `--without-python` flag

Even with the `--without-python` flag, checks were still done to ensure
the correct version of Python was being used. This commit fixes this so
these checks are not performed when `--without-python` is enabled.

Change-Id: I2242f2971a49ef28cff229ad0337bce0a998413d
Issue-on: https://gem5.atlassian.net/browse/GEM5-880
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39715
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Lukas Steiner <lsteiner@rhrk.uni-kl.de>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>