Revert "edition 2021"

This reverts commit 036c540.

Author: djc

Cargo.toml

@@ -3,7 +3,7 @@ name = "async-global-executor"
 version = "2.1.0"
 authors = ["Marc-Antoine Perennou <[email protected]>"]
 description =  "A global executor built on top of async-executor and async-io"
-edition = "2021"
+edition = "2018"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/Keruspe/async-global-executor"
 homepage = "https://github.com/Keruspe/async-global-executor"