Bump scipp from 25.5.1 to 25.8.0 in /requirements

Bumps [scipp](https://github.com/scipp/scipp) from 25.5.1 to 25.8.0.
- [Release notes](https://github.com/scipp/scipp/releases)
- [Commits](scipp/scipp@25.05.1...25.08.0)

---
updated-dependencies:
- dependency-name: scipp
  dependency-version: 25.8.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements/base.txt

@@ -138,7 +138,7 @@ sciline==25.5.2
     # via
     #   -r base.in
     #   essreduce
-scipp==25.5.1
+scipp==25.8.0
     # via
     #   -r base.in
     #   essreduce