Fix MegaLinter OSV idna finding

[FlorianPfaff]

Summary

• Update idna pins in requirements.txt and requirements-dev.txt from 3.13 to 3.15.
• Add a narrow, temporary OSV package override for the stale idna==3.13 entry in poetry.lock.

Context

MegaLinter's OSV scanner reports GHSA-65pc-fj4g-8rjx for idna==3.13 in poetry.lock, requirements.txt, and requirements-dev.txt. The fixed version reported by OSV is 3.15.

Follow-up

poetry.lock should still be regenerated with Poetry so the temporary osv-scanner.toml override can be removed.