Bump tornado from 6.4 to 6.4.1 in /images/singleuser-sample

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.4 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.4.0...v6.4.1)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: indirect
...

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

Author: dependabot[bot]

images/singleuser-sample/requirements.txt

@@ -101,7 +101,6 @@ jsonpointer==2.4
     # via jsonschema
 jsonschema[format-nongpl]==4.22.0
     # via
-    #   jsonschema
     #   jupyter-events
     #   jupyterlab-server
     #   nbformat
@@ -287,7 +286,7 @@ terminado==0.18.1
     #   jupyter-server-terminals
 tinycss2==1.3.0
     # via nbconvert
-tornado==6.4
+tornado==6.4.1
     # via
     #   ipykernel
     #   jupyter-client