Bump scipp from 25.3.0 to 25.4.0 in /requirements

Bumps [scipp](https://github.com/scipp/scipp) from 25.3.0 to 25.4.0.
- [Release notes](https://github.com/scipp/scipp/releases)
- [Commits](scipp/scipp@25.03.0...25.04.0)

---
updated-dependencies:
- dependency-name: scipp
  dependency-version: 25.4.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements/base.txt

@@ -139,7 +139,7 @@ sciline==24.10.0
     # via
     #   -r base.in
     #   essreduce
-scipp==25.3.0
+scipp==25.4.0
     # via
     #   -r base.in
     #   essreduce