Switch to Poetry /OR/ pin all the dependencies

[nolar]

Context:

I am getting annoyed by permanent CI/CD failures due to external releases, sometimes happening every few days in a row:

• CI/CD broken with Coverage 5.0 #272 (coveralls-vs-coverage)
• CI/CD broken with Kubernetes 1.17.0 & Minikube 1.6.0 due to RBAC #269 (k8s 1.17 + rbac in minikube)
• Ensure predictable mypy's type-checking rules in CI/CD #265 (mypy logic changes)
• Fix mypy's recommendations for __exit__ results #208 (mypy logic changes)

Goal

• Switch to Poetry for CI/CD dependencies.
• Lock all the dependencies' versions.
• Establish the auto-upgrade via the automatic PR creation & testing.

Optional (go the full journey with Poetry):

• Build & upload with Poetry instead of twine/setuptools.
• Detect versions from GitHub releases (previously done by setuptools_scm).

See python-poetry/poetry#366 for suggestions on the Poetry CI/CD templates (all of them use the virtualenv installation though, which is recommended to not be used).

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Plan B:

• Just pin all the dependencies in requirements.txt (manually).
• But still establish an auto-upgrader.