Add extension-module feature for pyo3

Apparently required to get the linkage correct.

Cargo.toml

@@ -22,11 +22,13 @@ edition = "2021"
 name = "_lib"
 crate-type = ["cdylib", "rlib"]  # rlib included just for testing
 
+# TODO: gil-refs currently seems to be required, but it should be removed
+# when possible.
 [dependencies]
 enum-map = "2.7.3"
 itoa = "1.0.10"
 once_cell = "1.19.0"
-pyo3 = { version = "0.21.0-beta.0", features = ["gil-refs"] }
+pyo3 = { version = "0.21.0-beta.0", features = ["gil-refs", "extension-module"] }
 thiserror = "1.0.58"
 
 [dev-dependencies]