Update keras requirement from <3.0.0,>=2.0.0 to >=2.0.0,<4.0.0 in /tests

Updates the requirements on [keras](https://github.com/keras-team/keras) to permit the latest version.
- [Release notes](https://github.com/keras-team/keras/releases)
- [Commits](keras-team/keras@2.0.0...v3.5.0)

---
updated-dependencies:
- dependency-name: keras
  dependency-type: direct:production
...

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

tests/constraints.txt

@@ -27,4 +27,4 @@ jaxlib<=0.4.14
 torch>=1.13,<2.3
 kornia==0.7.0
 networkx<=3.1.0
-keras>=2.0.0,<3.0.0
+keras>=2.0.0,<4.0.0