Bump tornado from 4.5.3 to 6.4.2

Bumps [tornado](https://github.com/tornadoweb/tornado) from 4.5.3 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/v6.4.2/docs/releases.rst)
- [Commits](tornadoweb/tornado@v4.5.3...v6.4.2)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -1,4 +1,4 @@
-tornado==4.5.3  # http://www.tornadoweb.org/en/stable/releases.html
+tornado==6.4.2  # http://www.tornadoweb.org/en/stable/releases.html
 psycopg2==2.9.10  # http://initd.org/psycopg/articles/tag/release/
 sqlalchemy>=1.3,<1.4  # http://docs.sqlalchemy.org/en/latest/changelog/index.html
 netifaces>=0.10,<0.12  # https://bitbucket.org/al45tair/netifaces/src/