Poetry lockfile should not be ignored

[dokterbob]

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For some reason, both poetry lock files are not committed to the repo.

This causes unpredictable builds and is explicitly against best practices, see: https://python-poetry.org/docs/basic-usage/#committing-your-poetrylock-file-to-version-control

Note also: "If you do not want to give up the reproducibility and performance benefits, consider a regular refresh of poetry.lock to stay up-to-date and reduce the risk of sudden breakage for users."

As a bonus, this will also make poetry install run way faster.

Specifically, the following should be removed from .gitignore:

• poetry.lock

Possibly related: #1189