net-mgmt/py-napalm: Update to 3.3.1

• Modernize "do-test" target while I'm here.

Changelogs since 3.2.0:

https://github.com/napalm-automation/napalm/releases/tag/3.3.1
https://github.com/napalm-automation/napalm/releases/tag/3.3.0

MFH: 2021Q3
(cherry picked from commit 3c88c6ce28ef494cb233caddacb4a229e2e03379)