Bump dnspython from 2.3.0 to 2.6.1 in /backend

Bumps [dnspython](https://github.com/rthalley/dnspython) from 2.3.0 to 2.6.1.
- [Release notes](https://github.com/rthalley/dnspython/releases)
- [Changelog](https://github.com/rthalley/dnspython/blob/main/doc/whatsnew.rst)
- [Commits](https://github.com/rthalley/dnspython/compare/v2.3.0...v2.6.1)

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

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

backend/requirements.txt

@@ -1,5 +1,5 @@
 certifi==2022.12.7
-dnspython==2.3.0
+dnspython==2.6.1
 eventlet==0.33.3
 python-engineio==4.3.4
 python-socketio==5.8.0