build(deps): update scikit-learn requirement in /requirements

Updates the requirements on [scikit-learn](https://github.com/scikit-learn/scikit-learn) to permit the latest version.
- [Release notes](https://github.com/scikit-learn/scikit-learn/releases)
- [Commits](scikit-learn/scikit-learn@1.5.0rc1...1.6.0)

---
updated-dependencies:
- dependency-name: scikit-learn
  dependency-type: direct:production
...

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

requirements/_tests.txt

@@ -18,5 +18,5 @@ fire ==0.7.*
 
 cloudpickle >1.3, <=3.1.0
 scikit-learn ==1.2.*; python_version < "3.9"
-scikit-learn ==1.5.*; python_version > "3.8"  # we do not use `> =` because of oldest replcement
+scikit-learn ==1.6.*; python_version > "3.8"  # we do not use `> =` because of oldest replcement
 cachier ==3.1.2