Bump mysql-connector-python from 8.2.0 to 9.1.0 in /backend

Bumps [mysql-connector-python](https://github.com/mysql/mysql-connector-python) from 8.2.0 to 9.1.0.
- [Changelog](https://github.com/mysql/mysql-connector-python/blob/trunk/CHANGES.txt)
- [Commits](mysql/mysql-connector-python@8.2.0...9.1.0)

---
updated-dependencies:
- dependency-name: mysql-connector-python
  dependency-version: 9.1.0
  dependency-type: direct:production
...

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Author: dependabot[bot]

backend/requirements.txt

@@ -1,4 +1,4 @@
 Flask==2.3.3
 flask-cors==4.0.0
-mysql-connector-python==8.2.0
+mysql-connector-python==9.1.0
 python-dotenv==1.0.0