backend: Update jputils requirements version ranges

Restrict 'jupyter-client' version to be earlier than 7 to workaround
upstream discrepancies.

Refs jupyter/jupyter_client#637

Signed-off-by: Ilias Katsakioris <elikatsis@arrikto.com>

backend/setup.py

@@ -36,15 +36,15 @@
         'kfp',
         'autopep8 >=1.4, <1.5',
         'astor >= 0.8.1',
-        'nbformat >=4.4, <5.0',
         'networkx >=2.3, <3.0',
         'jinja2 >=2.10, <3.0',
         'pyflakes >=2.1.1',
         'dill >=0.3, <0.4',
         'IPython >= 7.6.0',
-        'jupyter-client >= 5.3.4',
+        'jupyter-client >=5.3.4, <7.0.0',
         'jupyter-core >= 4.6.0',
-        'nbconvert >= 5.6.1, < 6.0.0',
+        'nbconvert >=5.6.1, <6.0.0',
+        'nbformat >=4.4, <5.0.0',
         'ipykernel >= 5.1.4',
         'notebook >= 6.0.0',
         'packaging > 20',