`python-language-server` deprecated, switch to `python-lsp-server`.

## Summary
[python-language-server](https://github.com/palantir/python-language-server) on which this Atom package depends is deprecated in favor of [python-lsp-server](https://github.com/python-lsp/python-lsp-server), as confirmed by this [issue](https://github.com/palantir/python-language-server/issues/935) and the long list of projects mentioning it underneath.

- The installation instructions for ide-python still mention installing the deprecated repo.
- The plugin defaults to the old version if the newer one is not found, as https://github.com/atom-community/ide-python/pull/389 shows.

## Motivation
- Allow future PLSP features to improve this package while keeping an up-to-date Langage Server backend.

## Describe alternatives you've considered
I just installed the correct version, and everything worked as expected.

首先查看 issue 中描述的安装说明和插件默认选择行为，然后比较 pull request 389 中的处理方式。将两个引用都从 python-language-server 更新为 python-lsp-server，并验证安装较新的 server 是否如报告所述正常工作。

由索引模型根据 Issue 内容生成。