build(deps): bump nbgitpuller in /images/singleuser-sample

Bumps [nbgitpuller](https://github.com/jupyterhub/nbgitpuller) from 1.0.2 to 1.2.0.
- [Changelog](https://github.com/jupyterhub/nbgitpuller/blob/main/CHANGELOG.md)
- [Commits](jupyterhub/nbgitpuller@1.0.2...1.2.0)

---
updated-dependencies:
- dependency-name: nbgitpuller
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

images/singleuser-sample/requirements.txt

@@ -2,4 +2,4 @@
 # .github/dependabot.yaml.
 
 jupyterhub==2.0.2
-nbgitpuller==1.0.2
+nbgitpuller==1.2.0