Turn off parallel docs build and pin dependencies

[coruscating]

Summary

The docs build is suddenly breaking with

  File "/home/runner/work/qiskit-experiments/qiskit-experiments/.tox/docs-parallel/lib/python3.8/site-packages/sphinx/util/parallel.py", line 126, in _join_one
    raise SphinxParallelError(*result)
sphinx.errors.SphinxParallelError: sphinx.errors.ExtensionError: Unable to find kernel (exception: No such kernel named python3)

Turning off the parallel flag didn't solve the problem, but pinning ipykernel and jupyter-client, both of which put out new releases yesterday, solved the problem. I'm keeping the parallel flag off since it tends to cause issues until this compatibility issue is resolved.

[nkanazawa1989]

LGTM

[wshanks]

I wonder if there is something unique to our set up here? I set up a new environment today and found that I still had to roll these libraries back even seven months later, and I don't see anyone else complaining about Unable to find kernel on the jupyter-sphinx repo (except here but that one does not mention pinning jupyter-client and ipykernel, just installing ipykernel; I hit the error with the current latest ipykernel).