Bump tornado from 5.1 to 6.3.2 in /docs/source

Bumps [tornado](https://github.com/tornadoweb/tornado) from 5.1 to 6.3.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v5.1.0...v6.3.2)

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

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

docs/source/requirements.txt

@@ -28,7 +28,7 @@ sphinx-autobuild==0.7.1
 sphinx-intl==0.9.11
 sphinx-rtd-theme==0.4.2
 sphinxcontrib-websupport==1.0.1
-tornado==5.1
+tornado==6.3.2
 typing==3.6.2
 urllib3
 watchdog==0.8.3