Log Message:
py-nbformat: update to 4.4.0

4.4.0:
- Explicitly state that metadata fields can be ignored.
- Introduce official jupyter namespace inside metadata (``metadata.jupyter``).
- Introduce ``source_hidden`` and ``outputs_hidden`` as official front-end
  metadata fields to indicate hiding source and outputs areas. **NB**: These
  fields should not be used to hide elements in exported formats.
- Fix ending the redundant storage of signatures in the signature database.
- :func:`nbformat.validate` can be set to not raise a ValidationError if
  additional properties are included.
- Fix for errors with connecting and backing up the signature database.
- Dict-like objects added to NotebookNode attributes are now transformed to be
  NotebookNode objects; transformation also works for `.update()`.