Pin ipykernel version (#253)

Author: MM0hsin

pyproject.toml

@@ -46,7 +46,7 @@ source = "https://github.com/stfc/aiida-mlip"
 [dependency-groups]
 dev = [
     "coverage[toml]<8.0.0,>=7.4.1",
-    "ipykernel>=6.30.0",
+    "ipykernel<7.1.0, >=6.30.0",
     "pgtest<2.0.0,>=1.3.2",
     "pytest<9.0,>=8.0",
     "pytest-cov<5.0.0,>=4.1.0",