A Union Traitlet
Allow for trait types like Type or Instance
Test for Union Traitlet
validate -> _validate
resolve classes
Overload __or__ on traitlets and add a test
Some cleanup unused code and missig use-strict
update to use event
some extra comma/semicolon cleanup
Merge pull request #6925 from SylvainCorlay/union

A union traitlet type
Merge pull request #6748 from Carreau/cleanup-nb

Some cleanup unused code and missing use-strict
handle various permission failures

- turn various EPERM, EACCES errors into 403
- add `writable` bool field to contents models
- if a notebook is not `writable`, save is disabled
s/open_w/atomic_writing/