support nbconvert version 6 when released

[chrisjsewell]

nbconvert 6.0 is in the works: jupyter/nbconvert#1045

One thing this will definitely break is the Jinja template names (jupyter/nbconvert#1056), which are changing to e.g. display_priority.tpl -> display_priority.j2 (see also jupyter/nbconvert#1119)

[MSeal]

FYI we're planning on doing an alpha release soon (this weekend perhaps) with the templating and python 3 changes that are part of the 6.0 release plan so you can get early access to the changes.

[chrisjsewell]

Thanks for the heads up @MSeal 😃 I will make sure to check it out

[MSeal]

nbconvert 6.0.0a0 is now available via pip install --pre nbconvert. This has the python 3 only changes to setup and the new template directories.