use nbconvert 5.6.1 until nbgrader templates are updated

[ttimbers]

The issue seems to be a broader issue with how nbconvert templates are now handled in version 6 and higher. I encountered this last term, and suspected as much, and now have confirmed.

The issue is that nbgrader has not updated their templates yet to be compatible with nbconvert v6 or higher (see here). The the interim solution is to downgrade nbconvert to version 5.6.1. Long-term, someone will need to update those templates...

It is affecting other projects as well, as it is vocally discussed in the nbconvert repo:

• nbconvert 6.0.0 causing build failures due to TemplateNotFound error jupyter/nbconvert#1369
• TemplateNotFound error when extending built-in template jupyter/nbconvert#1394