build: bump transformers test dep upper bound from <5 to <6

Transformers v5.x is released and our integration tests were stuck on
v4.57.6 due to the <5 cap in [test] extras. Some test failures in the
transformers bnb suite are already fixed in v5.x (see transformers#44604).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Titus-von-Koeller

pyproject.toml

@@ -69,7 +69,7 @@ test = [
     "lion-pytorch==0.2.3",
     "pytest~=8.3",
     "scipy>=1.11.4,<2",
-    "transformers>=4.30.1,<5"
+    "transformers>=4.30.1,<6"
 ]
 
 [tool.setuptools]