Bump tornado from 6.3.3 to 6.4

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>

Author: dependabot[bot]

requirements.txt

@@ -1 +1 @@
-tornado==6.3.3
+tornado==6.4

setup.py

@@ -33,6 +33,6 @@
         "Creative Commons Attribution-NoDerivatives 4.0 International"
         " (CC BY-ND 4.0)"
     ),
-    install_requires=["tornado==6.3.3"],
+    install_requires=["tornado==6.4"],
     python_requires=">=3.7"
 )