Bump tornado from 4.2.1 to 6.3.2

Bumps [tornado](https://github.com/tornadoweb/tornado) from 4.2.1 to 6.3.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](https://github.com/tornadoweb/tornado/compare/v4.2.1...v6.3.2)

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

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

requirements.txt

@@ -42,6 +42,6 @@ simplegeneric==0.8.1
 six==1.9.0
 sklearn==0.0
 terminado==0.5
-tornado==4.2.1
+tornado==6.3.2
 traitlets==4.0.0
 wheel==0.38.1