Force Cargo to use PyO3 0.20.2

PyO3 0.20.3 breaks building wheels with the default maturin GitHub
action.

Cargo.toml

@@ -20,7 +20,7 @@ anyhow = "1.0.80"
 chrono = "0.4.34"
 indexmap = "2.2.3"
 nom = "7.1.3"
-pyo3 = { version = "0.20.2", features = ["chrono"] }
+pyo3 = { version = "=0.20.2", features = ["chrono"] }
 rayon = "1.8.1"
 regex = "1.10.3"
 serde = { version = "1.0.197", features = ["derive"] }