refresh binder, update jupyter_lsp spec with install instructions

[bollwyvl]

Hello there!

This hoists the binder config back up to root, and updates the spec to be detected by jupyter_lsp.

It's testable here:

I've also updated the spec to not gobble up plain, as it's causing issues over on https://github.com/krassowski/jupyterlab-lsp/pull/493 because we kinda had the assumption under test that nobody would take it (whoops) and added the install instructions, which would propagate to our docs. There's a spot for extensions, so it could be nice to add robocop there.

It may also be worth looking at some other stuff in setup.py, like aligning the trove classifiers (which show up on PyPI) and the python_requires (which newer pips obey) to have a consistent story.

Anyhow, the binder feels pretty good: selenium stuff works (with headlessfirefox, didn't try too hard to get chromium working), debugger works, robocop works. Not blocking here, but hopefully we'll have a robotkernel soon which supports RF4 (which still has some nice interactive features over xeus-python)... and hopefully a working tests on jupyterlab-lsp! If the notebook issue can be resolved, then we could probably take the spec management burden off of this repo altogether!

[fabioz]

Just merged. Thanks for the fixes!