Relax the bounds on Send for JoinHandle impl
diff --git a/src/join_handle.rs b/src/join_handle.rs
index b3c4da5..b192418 100644
--- a/src/join_handle.rs
+++ b/src/join_handle.rs
@@ -25,7 +25,7 @@
 }
 
 unsafe impl<R: Send, T> Send for JoinHandle<R, T> {}
-unsafe impl<R: Send, T> Sync for JoinHandle<R, T> {}
+unsafe impl<R, T> Sync for JoinHandle<R, T> {}
 
 impl<R, T> Unpin for JoinHandle<R, T> {}