Drop Python 2.7 from test automation
Drop Python 2.7 from test automation

We do not need to include Python 2.7 versions of our tox environments any
more.

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

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