Bump tornado from 4.5.1 to 6.3.3

Bumps [tornado](https://github.com/tornadoweb/tornado) from 4.5.1 to 6.3.3.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v4.5.1...v6.3.3)

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

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

requirements.txt

@@ -21,6 +21,6 @@ Sphinx==1.6.2
 sphinx-autobuild==0.6.0
 sphinx-rtd-theme==0.2.4
 sphinxcontrib-websupport==1.0.1
-tornado==4.5.1
+tornado==6.3.3
 urllib3==1.26.5
 watchdog==0.8.3