`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.

Beginnen Sie damit, die in der Issue beschriebenen Installationsanweisungen und das Verhalten bei der Standardauswahl des Plugins zu überprüfen, und vergleichen Sie anschließend den Ansatz in Pull Request 389. Aktualisieren Sie beide Verweise von python-language-server auf python-lsp-server und überprüfen Sie, dass die Installation des neueren Servers wie berichtet funktioniert.

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