Update tornado requirement from >=6.0.3,~=6.0 to ~=6.4

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@v6.0.3...v6.4.0)

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

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

requirements.txt

@@ -1,5 +1,5 @@
 ovos-bus-client~=0.0, >=0.0.7
 ovos-utils~=0.0, >=0.0.37
 ovos-config~=0.0,>=0.0.12
-tornado~=6.0, >=6.0.3
+tornado~=6.4
 ovos-plugin-manager>=0.0.24