Bump tornado from 6.3.3 to 6.4 in /server

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.3.3 to 6.4.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.3.3...v6.4.0)

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

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

server/requirements.txt

@@ -1,4 +1,4 @@
-tornado==6.3.3
+tornado==6.4
 tornado-sqlalchemy==0.7.0
 datetime==4.9
 python-dateutil==2.8.2