Update parking_lot requirement from 0.8.0 to 0.9.0 (#232)

Updates the requirements on [parking_lot](https://github.com/Amanieu/parking_lot) to permit the latest version.
- [Release notes](https://github.com/Amanieu/parking_lot/releases)
- [Changelog](https://github.com/Amanieu/parking_lot/blob/master/CHANGELOG.md)
- [Commits](Amanieu/parking_lot@0.8.0...0.9.0)

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

Cargo.toml

@@ -18,7 +18,7 @@ ethereum-types = "0.6.0"
 futures = "0.1.26"
 jsonrpc-core = "11.0.0"
 log = "0.4.6"
-parking_lot = "0.8.0"
+parking_lot = "0.9.0"
 rustc-hex = "2.0.1"
 serde = { version = "1.0.90", features = ["derive"] }
 serde_json = "1.0.39"