build: Migrate to Poetry

Poetry is starting to be the standard used for the rest of Toolforge
infrastructure written in Python, and the lock file is a nice feature
for a fully containerized app. This commit migrates the app to use
Poetry. Dependencies are pinned to the currently running versions and a
follow-up will upgrade them.

Bug: T354751

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Related MRs:

• !54 (merged)
• !53 (merged) (This MR)
• !55 (merged)
• main