Configure pytest to fail on warnings

[abey79]

What

This PR configures pytest such that it fails if a warning is emitted (e.g. a deprecation warning).

• add the relevant pytest config in our pyproject.toml
• add -c path/to/pyproject.toml in pytest invocations (justfile and CI)
• fix ugly two instances of "file not closed" warnings

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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[Wumpf]

any more context why this one is ignored?

[abey79]

The context is that I've been lazy and discarded this error as what I initially (mis-)read as some internal Pytest stuff. It turns out they were caused by two nasty instance of us not closing files properly, which I now fixed.

Thanks for not letting me be lazy.

[Wumpf]

neat!