Add linker directive for *-darwin to fix PyO3

Without these flags I get linking errors like this when building from
nixpkgs:

  = note: Undefined symbols for architecture arm64:
            "_PyExc_ConnectionResetError", referenced from:
                pyo3::type_object::PyTypeInfo::type_object::hd73bc9c256c52717 in libpyo3-c59aeb8b74bb57e0.rlib(pyo3-c59aeb8b74bb57e0.pyo3.e60578ba-cgu.0.rcgu.o)

See: https://pyo3.rs/main/building_and_distribution#macos

.cargo/config.toml

@@ -0,0 +1,11 @@
+[target.x86_64-apple-darwin]
+rustflags = [
+  "-C", "link-arg=-undefined",
+  "-C", "link-arg=dynamic_lookup",
+]
+
+[target.aarch64-apple-darwin]
+rustflags = [
+  "-C", "link-arg=-undefined",
+  "-C", "link-arg=dynamic_lookup",
+]