Bump idna from 3.10 to 3.15 in /backend

Bumps [idna](https://github.com/kjd/idna) from 3.10 to 3.15.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.md)
- [Commits](kjd/idna@v3.10...v3.15)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -96,7 +96,7 @@ humanize==4.13.0
     # via flower
 ianalyzer-readers==0.4.0
     # via -r requirements.in
-idna==3.10
+idna==3.15
     # via requests
 iniconfig==2.1.0
     # via pytest