Update parking_lot requirement from 0.11 to 0.12

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@0.11.0...0.12.0)

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

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Author: dependabot[bot]

tokio-postgres/Cargo.toml

@@ -46,7 +46,7 @@ byteorder = "1.0"
 fallible-iterator = "0.2"
 futures = "0.3"
 log = "0.4"
-parking_lot = "0.11"
+parking_lot = "0.12"
 percent-encoding = "2.0"
 pin-project-lite = "0.2"
 phf = "0.10"