chore: bump tornado from 6.3.3 to 6.4.1 in /images/mkdocs_plus

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.3.3 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.3.3...v6.4.1)

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

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

images/mkdocs_plus/requirements.txt

@@ -35,6 +35,6 @@ six==1.16.0
 smmap2==3.0.1
 soupsieve==2.3.1
 tinycss2==1.1.1
-tornado==6.3.3
+tornado==6.4.1
 WeasyPrint==53.4
 webencodings==0.5.1