Bump scikit-learn from 1.3.2 to 1.4.0 in /testdata

Bumps [scikit-learn](https://github.com/scikit-learn/scikit-learn) from 1.3.2 to 1.4.0.
- [Release notes](https://github.com/scikit-learn/scikit-learn/releases)
- [Commits](scikit-learn/scikit-learn@1.3.2...1.4.0)

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

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

testdata/requirements.txt

@@ -1,5 +1,5 @@
 xgboost==1.6.0
-scikit-learn==1.3.2
+scikit-learn==1.4.0
 treelite==3.9.1
 treelite-runtime==3.9.1
 pandas==2.2.0