Add extra metadata to setup.py
Add extra metadata to setup.py

Add the license and the content type of long_description. Add extra
supported versions of Python to the Classifiers list.

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

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