Bump python-dotenv from 0.14.0 to 0.15.0 in /backend

Bumps [python-dotenv](https://github.com/theskumar/python-dotenv) from 0.14.0 to 0.15.0.
- [Release notes](https://github.com/theskumar/python-dotenv/releases)
- [Changelog](https://github.com/theskumar/python-dotenv/blob/master/CHANGELOG.md)
- [Commits](theskumar/python-dotenv@v0.14.0...v0.15.0)

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Author: dependabot[bot]

backend/requirements.txt

@@ -3,7 +3,7 @@ gunicorn==20.0.4
 
 flask-cors==3.0.9
 
-python-dotenv==0.14.0
+python-dotenv==0.15.0
 
 Geohash==1.0
 python-geohash==0.8.5