[DOC] Pin jupyter-client

[barry-jin]

Description

Fix website build by pinning jupyter-client to the working version.
Reference:
https://jenkins.mxnet-ci.amazon-ml.com/blue/organizations/jenkins/mxnet-validation%2Fwebsite/detail/PR-20543/3/pipeline
jupyter/jupyter_client#637

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[mxnet-bot]

Hey @barry-jin , Thanks for submitting the PR
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