Update plugin to use python-lsp-server instead of python-language-server

[andfoy]

Hi,

Given that Palantir removed all the community maintenance for both python-language-server (pyls) and python-jsonrpc-server (pyls_jsonrpc), we decided to create a separate organization that now maintains a community-based fork for the aforementioned packages, which can be found at: https://github.com/python-lsp/python-lsp-server and https://github.com/python-lsp/python-lsp-jsonrpc.

Our recommendation is to migrate this plugin to be compatible with the community server instead of the Palantir one. In order to do so, here is a migration guide that should help:

1. The plugin should replace python-language-server by python-lsp-server on its requirements.
2. The pyls namespace is now pylsp, which means that all pyls imports should be replaced by pylsp.
3. The pyls_* hookspecs are now pylsp_* hookspecs.
4. The plugin can preserve the pyls_ prefix on its namespace in order to prevent further administration and release issues that may arise from renaming it.

Feel free to ask any question about the migration process or about the new community organization for the server.

[haplo]

@rupert Is there a chance you could review #37, merge it and release a python-lsp-server-compatible version to PyPI? That would be great for all users of the plugin.