Update tornado requirement from >=6.5.5 to >=6.5.6

Updates the requirements on [tornado](https://github.com/tornadoweb/tornado) to permit the latest version.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.5.5...v6.5.6)

---
updated-dependencies:
- dependency-name: tornado
  dependency-version: 6.5.6
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -1 +1 @@
-tornado>=6.5.5
+tornado>=6.5.6