Nbformat

- Starting with 5.0, `nbformat` is now Python 3 only (>= 3.5)
- Add execution timings in code cell metadata for v4 spec.
`"metadata": { "execution": {...}}` should be populated with
kernel-specific timing information.
- Documentation for how markup is used in notebooks added
- Link to json schema docs from format page added
- Documented the editable metadata flag
- Update description for collapsed field
- Documented notebook format versions 4.0-4.3 with accurate json
schema specification files
- Clarified info about name's meaning for cells
- Added a default execution_count of None for
new_output_cell('execute_result')
- Added support for handling nbjson kwargs
- Wheels now correctly have a LICENSE file
- Travis builds now have a few more execution environments

- 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.
- `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 \[.update()\]{.title-ref}.

- A new pluggable `SignatureStore` class allows specifying different
ways to record the signatures of trusted notebooks. The default is
still an SQLite database. See
`pluggable_signature_store` for more
information.
- `nbformat.read` and
`nbformat.write` accept file paths as
bytes as well as unicode.
- Fix for calling `nbformat.validate`
on an empty dictionary.
- Fix for running the tests where the locale makes ASCII the default
encoding.
- Include nbformat-schema files (v3 and v4) in nbformat-schema npm
package.
- Include configuration for appveyor's continuous integration
service.

- Update nbformat spec version to 4.2, allowing JSON outputs to have
any JSONable type, not just `object`, and mime-types of the form
`application/anything+json`.
- Define basics of `authors` in notebook metadata.
`nb.metadata.authors` shall be a list of objects with the property
`name`, a string of each author's full name.
- Update use of traitlets API to require traitlets 4.1.
- Support trusting notebooks on stdin with
`cat notebook | jupyter trust`