chore(cargo): update serial_test requirement from ^2.0 to ^3.1

Updates the requirements on [serial_test](https://github.com/palfrey/serial_test) to permit the latest version.
- [Release notes](https://github.com/palfrey/serial_test/releases)
- [Commits](palfrey/serial_test@v2.0.0...v3.1.1)

---
updated-dependencies:
- dependency-name: serial_test
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -33,4 +33,4 @@ default-features = false
 features = []
 
 [dev-dependencies.serial_test]
-version = "^2.0"
+version = "^3.1"