Import importnb to this project

[importnb](https://github.com/deathbeds/importnb) is a newer and better version of this package, that does the same things - but is more maintained.

I spoke to @tonyfast and there is interest in making this happen. It will help adoption, and keep this project moving forward.

Things to do:
- [ ] Import contents of that git repo to here, preserving all histories
- [ ] Add maintainers of importnb to this repo
- [ ] Add maintainers of importnb to pypi
- [ ] Make an announcement

The issue names no files, tests, or entry points. Start by reviewing the linked importnb repository and the four checklist items; done means its contents and histories are imported, maintainers are added to the repository and PyPI, and an announcement is made.

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