Update to Python 3.7 and 2020 Kubernetes cluster

The changes to the deployment.yaml are mostly based on those in stashbot
of rLTSTac90ab6af724 [1] and rLTSTf1ad69c461a1 [2].

[1]: https://phabricator.wikimedia.org/rLTSTac90ab6af724a44c30fd2e36a8bf3460fb1fabd7
[2]: https://phabricator.wikimedia.org/rLTSTf1ad69c461a14f385c5f048f3abe92add09f37c5