Bump idna from 3.7 to 3.15 in /requirements

Bumps [idna](https://github.com/kjd/idna) from 3.7 to 3.15.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.md)
- [Commits](kjd/idna@v3.7...v3.15)

---
updated-dependencies:
- dependency-name: idna
  dependency-version: '3.15'
  dependency-type: indirect
...

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Author: dependabot[bot]