Update itertools requirement from 0.11.0 to 0.12.0

Updates the requirements on [itertools](https://github.com/rust-itertools/itertools) to permit the latest version.
- [Changelog](https://github.com/rust-itertools/itertools/blob/master/CHANGELOG.md)
- [Commits](rust-itertools/itertools@v0.11.0...v0.12.0)

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

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

Cargo.toml

@@ -27,7 +27,7 @@ bytes = { version = "1.2.1", optional = true }
 serde = { version = "1.0", features = ["derive"] }
 assert_matches = "1.5.0"
 generator = "0.7"
-itertools = "0.11.0"
+itertools = "0.12.0"
 
 [package.metadata.docs.rs]
 all-features = true