Bump requests from 2.28.2 to 2.31.0 in /backend

Bumps [requests](https://github.com/psf/requests) from 2.28.2 to 2.31.0.
- [Release notes](https://github.com/psf/requests/releases)
- [Changelog](https://github.com/psf/requests/blob/main/HISTORY.md)
- [Commits](psf/requests@v2.28.2...v2.31.0)

---
updated-dependencies:
- dependency-name: requests
  dependency-type: direct:production
...

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

backend/requirements.txt

@@ -22,7 +22,7 @@ pytest-sqlalchemy-mock==0.1.4
 python-dotenv==1.0.0
 pytz==2023.2
 PyYAML==6.0
-requests==2.28.2
+requests==2.31.0
 requests-mock==1.10.0
 slack_bolt==1.17.0
 slack_sdk==3.20.2