Jupyter-ui-poll

Maintenance release

- set dependency constraint `ipykernel<6`

Changes in `ipykernel` going to version 6 require switching to async mode of operation.

Maintenance release

- Packaging related changes
- Push to test pypi from master
- Push to pypi on release

- Fix a bug, where output would sometimes go to the wrong cell when using Run All mode
- Improve docs and sample notebooks
- Add Binder launcher

It's a usable library now.

- Better docs
- More ways to work: callback, iterator and pure poll
- Better handling of exceptions raised during replay

Testing release process