Use jupyter-client<7 due to issue with nbconvert (BlueBrain/nmodl#722)

try suggestion in jupyter/jupyter_client#637 (comment)

NMODL Repo SHA: BlueBrain/nmodl@097856b

Author: olupton

setup.py

@@ -124,6 +124,7 @@ def _config_exe(exe_name):
     zip_safe=False,
     setup_requires=[
         "jinja2>=2.9.3",
+        "jupyter-client<7", # try and work around: TypeError in notebooks/nmodl-kinetic-schemes.ipynb: 'coroutine' object is not subscriptable
         "jupyter",
         "m2r",
         "mistune<2",  # prevents a version conflict with nbconvert