add test_connect file missed in last backport
Backport PR #3939: minor checkpoint cleanup

- remember list of checkpoints browser-side
- don't clobber list when a new checkpoint is created
- cleanup references in MenuBar restore list.
  There was a closure issue, where multiple menu items would actually all
  restore the same checkpoint.

Issues revealed by rgbkrk/bookstore, which supports multiple checkpoints.

I'm fine if this doesn't get into 1.0, given timing.
Backport PR #3960: Don't make sphinx a dependency for importing nbconvert

Just move the import so it doesn't fire unless sphinx is used.

Even though it's trivial, probably not for 1.0.