Unnecessary Send requirement on Sleeper::Sleep?

[jessa0]

I'm trying to use backoff::future::Retry using gloo-timers::future::TimeoutFuture as Sleeper::Sleep on WASM/Web, which is !Send. Sleeper currently requires type Sleep to be Send, but it seems this is not strictly necessary for the operation of Retry. I do understand, however, it'd technically be a visible API change to remove the Send bound, since the trait is public and not sealed, but it doesn't seem like a breaking change for the intended use of the Sleeper trait.