Bump tornado from 5.0 to 6.3.3 in /docs (#528)

Bumps [tornado](https://github.com/tornadoweb/tornado) from 5.0 to 6.3.3.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v5.0.0...v6.3.3)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -4,6 +4,6 @@ Jinja2==2.7.1
 livereload==2.5.1
 Markdown==3.0.0
 PyYAML==5.1
-tornado==5.0
+tornado==6.3.3
 mdx_gh_links>=0.2
 markupsafe==2.0.1