Bump plopp from 25.5.0 to 25.6.0 in /requirements

Bumps [plopp](https://github.com/scipp/plopp) from 25.5.0 to 25.6.0.
- [Release notes](https://github.com/scipp/plopp/releases)
- [Commits](scipp/plopp@25.05.0...25.06.0)

---
updated-dependencies:
- dependency-name: plopp
  dependency-version: 25.6.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements/base.txt

@@ -50,7 +50,7 @@ packaging==25.0
     #   matplotlib
 pillow==11.1.0
     # via matplotlib
-plopp==25.5.0
+plopp==25.6.0
     # via -r base.in
 pydantic==2.10.6
     # via -r base.in