Bump tornado from 5.0.2 to 6.3.3 in /docs

Bumps [tornado](https://github.com/tornadoweb/tornado) from 5.0.2 to 6.3.3.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v5.0.2...v6.3.3)

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

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

docs/requirements-rtfd.txt

@@ -1,4 +1,4 @@
-tornado==5.0.2
+tornado==6.3.3
 peewee==3.2.2
 requests==2.25.1
 pycurl==7.43.0.1