Update travis configuration

[wisp3rwind]

• Bump python versions
• travis appears to use Xenial everywhere nowadays, so no need to force this distribution in order to get recent Python
• the sudo keyword was deprecated because travis now uses a fully virtualized infrastructure: link

[wisp3rwind]

Fails because nosetests is dead, and does not support Python 3.9, nose-devs/nose#1099

[wisp3rwind]

Tentatively, I replaced nose with pytest. I'm not super familiar with Python's testing ecosystem, so maybe that's not the best choice. There's at least also nose2. Any opinion on the test runner, @geigerzaehler?

[geigerzaehler]

Thanks for fixing this! pytest is fine with me, that is what I was using recently.

I think we may also want to update the envlist in tox.ini.

[wisp3rwind]

I think we may also want to update the envlist in tox.ini.

👍

[geigerzaehler]

🚀