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] <support@github.com>

tests/constraints.txt

@@ -26,7 +26,7 @@ jax<=0.4.14
 jaxlib<=0.4.14
 kornia==0.7.0
 networkx<=3.1.0
-keras>=2.0.0,<3.0.0
+keras>=2.0.0,<4.0.0
 
 --extra-index-url https://download.pytorch.org/whl/cpu
 torch>=1.13,<2.3