Upgrade to 2021 edition

Author: cuviper

Cargo.toml

@@ -11,7 +11,7 @@ repository = "https://github.com/rust-num/num-complex"
 version = "0.4.5"
 readme = "README.md"
 exclude = ["/ci/*", "/.github/*"]
-edition = "2018"
+edition = "2021"
 rust-version = "1.60"
 
 [package.metadata.docs.rs]