CI: update CI matrix to include latest python release
CI: update CI matrix to include latest python release

Run specialised builds (python-only and lint) on the latest version of
the interpreter.

Signed-off-by: Joshua Lock <edc229f50a11a11cdece0270af6320fb299dca1e@vmware.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib