Port to trio async library

[SlugFiller]

Changed from sync IO to async IO with cancellation scopes. This enables a few improvements that would have been difficult or impossible before:

• GPG agent allows multiple concurrent clients
• SSH agent no longer holds an exclusive lock for every request handled. Instead, the device is locked while being connected, only for the duration of operations done with the device.
• Clean handling of exit from the agents with Ctrl+C
• Clean handling of exit while device interaction or pinentry are running with Ctrl+C
• When the device requests interaction, a pinentry message is displayed. It automatically closes when the operation completes.
• It is possible to close the GPG agent with either the KILLAGENT message or Ctrl+C.
• More gracefully handle various errors in GPG agent, preventing agent crash, or frozen client.

[romanz]

Many thanks! Will try to review when I'll be available (hopefully in a few weeks).