Bump ujson from 5.4.0 to 5.12.1 in /examples/server/sanic

Bumps [ujson](https://github.com/ultrajson/ultrajson) from 5.4.0 to 5.12.1.
- [Release notes](https://github.com/ultrajson/ultrajson/releases)
- [Commits](https://github.com/ultrajson/ultrajson/compare/5.4.0...5.12.1)

---
updated-dependencies:
- dependency-name: ujson
  dependency-version: 5.12.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

examples/server/sanic/requirements.txt

@@ -4,5 +4,5 @@ python_engineio
 python_socketio
 sanic==20.12.7
 six==1.10.0
-ujson==5.4.0
+ujson==5.12.1
 uvloop==0.8.0