Bump pyjwt from 2.3.0 to 2.4.0 in /backend

Bumps [pyjwt](https://github.com/jpadilla/pyjwt) from 2.3.0 to 2.4.0.
- [Release notes](https://github.com/jpadilla/pyjwt/releases)
- [Changelog](https://github.com/jpadilla/pyjwt/blob/master/CHANGELOG.rst)
- [Commits](jpadilla/pyjwt@2.3.0...2.4.0)

---
updated-dependencies:
- dependency-name: pyjwt
  dependency-type: indirect
...

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Author: dependabot[bot]