ci: fix notebook tests

see jupyter/jupyter_client#637

Author: Simon Brugman

requirements-test.txt

@@ -1,6 +1,6 @@
 pytest>=4.0.2
 git+https://github.com/pytest-dev/pluggy.git
 pytest-notebook>=0.6.1
-jupyter_client>=5.2.3
+jupyter_client>=5.2.3,<7
 ipykernel>=5.1.3
 pre-commit>=2.9.0