Change link to python language server

[zerocewl]

the old python-language-server seems to be outdated / deprecated and might be replaced by a link to the current version / fork of python-lsp-server

python-lsp-server is a fork created by the community because I was the maintainer of this repo but lost my write permissions several months ago. So, it's not something "official", it's just what we needed to do out of necessity.

There's no one around to merge PRs, so it's not possible to add a note to the Readme. But I don't know if the Palantir guys plan to support this project later on their own (I doubt that, but who knows).

also see related issue comments in the old pyls repo:

palantir/python-language-server#901 (comment)
palantir/python-language-server#901 (comment)
palantir/python-language-server#935

[davidhalter]

Feel free to PR this.