move to 2021 edition

No code changes for now.

Author: sunshowers

Cargo.toml

@@ -7,7 +7,7 @@ repository = "https://github.com/diem/diem-devtools"
 license = "MIT OR Apache-2.0"
 publish = ["crates-io"]
 readme = "README.md"
-edition = "2018"
+edition = "2021"
 categories = ["development-tools::testing"]
 keywords = ["datatest", "data-driven-tests", "test-harness"]