917 Update jedi to v0.18.0+

[Vegemash]

This drops support for python 2 and 3.5 resolves #917

[palantirtech]

Thanks for your interest in palantir/python-language-server, @Vegemash! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[mattst88]

Anybody home?

Surely this doesn't require signing a CLA.

[ccordoba12]

We're creating a fork of this project because this one is unmaintained now:

https://github.com/python-lsp/python-lsp-server

Support for Jedi 0.18 was already added and it'll be released soon.