Improve CI/CD performance by caching poetry

[alexpovel]

Setting up poetry (10 - 20s) and the virtual environment (c. 2 minutes) can take a long time. It should be cached, and as key pyproject.toml and (mainly?) poetry.lock should be used. We have to set this up multiple times, so create a reusable action/workflow?

For example, when creating a release, all this has to run twice. Once for a commit itself, and right after for the new and merged/rebased release-please commit.

[alexpovel]

Done with #27 / 23dca4b.