Use community-maintained Python language server

[jinnovation]

The Palantir Python language server is unmaintained now (palantir/python-language-server#935) and work has shifted over to a community-maintained fork at https://github.com/python-lsp/python-lsp-server.

Once work concludes on their end to have plugins, e.g. for mypy and flake8, either update to support the new python-lsp-server or fork in order to do so (python-lsp/python-lsp-server#27), it'll be time for me to update my own config accordingly to purge all remnants of the former lang server.

At minimum, I'll need the following plugins:

• pyls-black
• pyls-isort
• pyls-mypy

[jinnovation]

All three of the cited plugins have now been updated w/ support for python-lsp-server. Nothing else blocking migration, as far as I can tell.