sanity: Bump min python version to 3.6

There are a number of reasons 3.6 is a good minimum version. Of our supported/tested
distros, only debian 9 still had python 3.5, the others have 3.6+ or already
required buildtools-tarball.

New versions of qemu need python 3.6 as a minimum. We could work around that
but it seems simper to require 3.6 which will allow other improvements.

As such, bump the minimum python version requirement to 3.6.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>