[feature request] Async call signature fetching

My biggest pet peeve with call signatures is the initial delay required for loading the module containing the desired function. This delay is fairly short, ~500ms on average, maybe 1s for Scipy stuff. However, it blocks visual updates, preventing the user from seeing what they type until the call signature can be displayed.

Ideally, the module loading should be done in asynchronously.

This is something I would be willing to implement myself. How much work would be required for such a change? I'm also not sure how this would tie in with the open PR #420 .