WIP Upgrade to JupyterLab 3

[jtpio]

Fixes #89

TODO

• [x] Switch to jupyter_packaging
• [x] Run python -m jupyterlab.upgrade_extension . in labextension/
• [x] Rename static to nbextension
• [ ] Upgrade the navigator
• [x] Update CI: make sure the classic notebook and lab extensions are correctly packaged
• [x] Update dev instructions

[coveralls]

Pull Request Test Coverage Report for Build 310924715

• 6 of 7 (85.71%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 57.986%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
jupyter_conda/init.py	6	7	85.71%
<!--	Total:	6	7	85.71%	-->

Totals
Change from base Build 278617331:	0.1%
Covered Lines:	743
Relevant Lines:	1143

---

💛 - Coveralls

[jtpio]

This is probably best to start over since the repo structure was changed in https://github.com/mamba-org/gator/pull/91