Switch to py38 as the primary CI version, keep py37 tested

[nolar]

What do these changes do?

Switch from Python 3.7 to 3.8 as the primary testing environment in Travis, and resolve few issues/warnings with 3.8. Python 3.7 compatibility is still tested and ensured.

Description

The testing is now organised this way:

• The latest Python — with all supported Kubernetes versions.
• The latest Kubernetes — with all supported Python versions (3.7 & 3.8 & optionally 3.9-dev).

We assume that if something breaks, it either breaks in the language (and one K8s version is enough to detect it), or in Kubernetes API (and one Python version is enough to detect it) — there is no need to test a real matrix "all Pythons vs. all K8ses".

Travis CI has a limit of 5 builds running in parallel (for us), so more than 5 or 10 builds delays the build results. Currently, we have 10 — and so it will be with the new grid (for a while).

Issues/PRs

Issues: #13

Type of changes

• Mostly CI/CD automation, contribution experience

Checklist

• The code addresses only the mentioned problem, and this problem only
• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt

[zincr]

🤖 zincr found 0 problems , 0 warnings

✅ Large Commits
✅ Approvals
✅ Specification
✅ Dependency Licensing

[zincr]

🤖 zincr found 1 problem , 0 warnings

❌ Approvals
✅ Large Commits
✅ Specification
✅ Dependency Licensing

Details on how to resolve are provided below

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Approvals

All proposed changes must be reviewed by project maintainers before they can be merged

Not enough people have approved this pull request - please ensure that 1 additional user, who have not contributed to this pull request approve the changes.

• ✅ Approved by PR author @nolar
• ❌ 1 additional approval needed