versioneer causes extraneous output in some docker containers

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.

Inspect Versioneer's version-check path and moldesign/_notebooks/nbscripts/strip_nb_output.py, then reproduce the warning in a development Docker container without nbformat. Done means version lookup no longer emits the ModuleNotFoundError or external-filter errors shown in the issue.

Written by the indexing model from the issue text.