[Draft] Notify the user when using the python-language-server backend

[guillaumealgis]

As discussed in #18, this adds a notification to the user when using the python-language-server package as a LSP server, instead of the more recent and maintained python-lsp-server.

• If clicking "Ignore", the notification will disappear and never be displayed again until the pyls.executable preference changes (allowing the users to keep using python-language-server if they choose to, without being bothered again in the future)
• If clicking "More info" a more details page of explanations is opened in a browser, currently the URL opened is Migrate to python-lsp-server ? #18, but we could change it for something more detailed, like a wiki page for example
• "OK" only dismisses the notification, but it will be shown again the next time the LSP server is started

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This is a draft, because:

1. This PR builds upon Migrate to python-lsp-server ? #18, so Migrate to python-lsp-server ? #18 needs to be merged before we can apply this
2. The "More info" URL and its content must be validated.

[mmshivesh]

I'm going ahead and merging this if there's been no issues so far.