actix-web-lab: v0.20.0

• Add sse::Sse::from_infallible_stream() method.
• Add sse::Sse::{from_receiver, from_infallible_receiver}() methods.
• Remove sse::{Sender, ChannelStream} types.
• Remove sse::{SendError, TrySendError} types.
• Remove sse::channel() function.
• Remove sse::{SseSendError, SseTrySendError, SseData, SseMessage} type aliases.
• Remove web::Redirect responder and web::redirect() function.
• Remove guard::Acceptable guard type.