Asyncio login

[tulir]

All the login stuff seems to be synchronous, which makes it hard to use properly from an asyncio context. Especially login needing callbacks (CredentialsPrompt) makes it nearly impossible.

The default prompt could still use the synchronous input even if the login stuff was asyncio, but custom asyncio prompts can't be used when the rest is synchronous.

[tdryer]

Right now auth is using mechanicalsoup which is synchronous-only. If we drop the automatic login method in the future, it should be easy enough to port to async.