versioneer causes extraneous output in some docker containers

[avirshup]

Versioneer, in checking for MDT's version number, causes these warnings to get spit into the logs:

ModuleNotFoundError: No module named 'nbformat'
error: external filter moldesign/_notebooks/nbscripts/strip_nb_output.py failed 1
error: external filter moldesign/_notebooks/nbscripts/strip_nb_output.py failed

This will only happen when working directly from a development repo, so it's low priority.