update edition to 2021

Signed-off-by: Petre Eftime <petre.eftime@gmail.com>

Author: petreeftime

Cargo.toml

@@ -3,7 +3,7 @@ description = "Interact with unix processes/bash the same way as pexpect or Don
 name        = "rexpect"
 version     = "0.4.0"
 authors     = ["Philipp Keller <philipp.keller@gmail.com>"]
-edition     = "2018"
+edition     = "2021"
 repository  = "https://github.com/philippkeller/rexpect"
 homepage    = "https://github.com/philippkeller/rexpect"
 license     = "MIT"