scripts: make python shebangs specific about desired version

The RPM packaging tools like to make sure that all packaged python
scripts have version-unambiguous shebangs. Be more specific about the
desired python version in a couple of places to avoid having to disable
these checks in make rpm-pkg.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>