Bump uvicorn from 0.20.0 to 0.29.0 in /backend

Bumps [uvicorn](https://github.com/encode/uvicorn) from 0.20.0 to 0.29.0.
- [Release notes](https://github.com/encode/uvicorn/releases)
- [Changelog](https://github.com/encode/uvicorn/blob/master/CHANGELOG.md)
- [Commits](encode/uvicorn@0.20.0...0.29.0)

---
updated-dependencies:
- dependency-name: uvicorn
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/requirements.txt

@@ -9,4 +9,4 @@ python-jose==3.3.0
 python-multipart==0.0.5
 sendgrid==6.9.7
 SQLAlchemy==1.4.42
-uvicorn==0.20.0
+uvicorn==0.29.0