Bump plopp from 24.9.1 to 24.9.2 in /requirements

Bumps [plopp](https://github.com/scipp/plopp) from 24.9.1 to 24.9.2.
- [Release notes](https://github.com/scipp/plopp/releases)
- [Commits](scipp/plopp@24.09.1...24.09.2)

---
updated-dependencies:
- dependency-name: plopp
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

requirements/base.txt

@@ -36,7 +36,7 @@ packaging==24.1
     # via matplotlib
 pillow==10.4.0
     # via matplotlib
-plopp==24.9.1
+plopp==24.9.2
     # via -r base.in
 pyparsing==3.1.4
     # via matplotlib