use fixed version of async-std

Author: amfern

Cargo.toml

@@ -20,8 +20,7 @@ packed_struct_codegen = "0.3"
 futures = "0.3.4"
 
 [dependencies.async-std]
-git = "https://github.com/async-rs/async-std"
-branch = "1-6-3"
+version = "1.6.3"
 features = ["unstable", "attributes"]
 
 [features]