Update syn requirement from ~1.0.13 to ~2.0.52

Updates the requirements on [syn](https://github.com/dtolnay/syn) to permit the latest version.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@1.0.13...2.0.52)

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

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

Cargo.toml

@@ -25,4 +25,4 @@ pyo3 = { version = ">=0.15, <0.21", default-features = false, features = ["auto-
 
 # `pyo3-macros` is lying about the minimal version for its `syn` dependency.
 # Because we're testing with `-Zminimal-versions`, we need to explicitly set it here.
-syn = "~1.0.13"
+syn = "~2.0.52"