Bump rustls to 0.21 (#50)

Author: mlemesle

Cargo.toml

@@ -23,7 +23,7 @@ appveyor = { repository = "async-std/async-tls" }
 [dependencies]
 futures-io = "0.3.5"
 futures-core = "0.3.5"
-rustls = "0.20.6"
+rustls = "0.21"
 rustls-pemfile = "1.0"
 webpki = { version = "0.22.0", optional = true }
 webpki-roots = { version = "0.22.3", optional = true }