Jupyter-core

[on GitHub](https://github.com/jupyter/jupyter_core/releases/tag/4.2.0)

- Make `jupyter` directory top
priority in search path for subcommands, so that
`jupyter-subcommand` next to
`jupyter` will always be picked if
present.
- Avoid using `shell=True` for subcommand dispatch on Windows.

[on GitHub](https://github.com/jupyter/jupyter_core/releases/tag/4.1.1)

- Include symlink directory and real location on subcommand PATH when
`jupyter` is a symlink.

[on GitHub](https://github.com/jupyter/jupyter_core/releases/tag/4.1.0)

- Add `jupyter.py` module, so that
`python -m jupyter` always works.
- Add prototype `jupyter troubleshoot` command for displaying
environment info.
- Ensure directory containing `jupyter` executable is included when
dispatching subcommands.
- Unicode fixes for Legacy Python.

[on GitHub](https://github.com/jupyter/jupyter_core/releases/tag/4.0.6)

- fix typo preventing migration when custom.css is missing