Revert/revisit pinning of nbconvert done in #733

[woodsp-ibm]

#732 noted a failure which manifested when nbconvert 7.14 was released. #733 pinned the version on nbconvert to get CI running again so the notebooks jobs would pass. The issue is just to serve as a reminder to re-visit this in the future e.g. if/when a newer nbconvert is released or some other finding perhaps indicates something needs to be done to the source notebooks even though things had seemingly been working until then.