fix either feature conditional compilation, again

newsfragments/3834.fixed.md

@@ -0,0 +1 @@
+Fix compilation failure with `either` feature enabled without `experimental-inspect` enabled.

src/conversions/either.rs

@@ -94,7 +94,13 @@ where
         } else if let Ok(r) = obj.extract::<R>() {
             Ok(Either::Right(r))
         } else {
-            let err_msg = format!("failed to convert the value to '{}'", Self::type_input());
+            // TODO: it might be nice to use the `type_input()` name here once `type_input`
+            // is not experimental, rather than the Rust type names.
+            let err_msg = format!(
+                "failed to convert the value to 'Union[{}, {}]'",
+                std::any::type_name::<L>(),
+                std::any::type_name::<R>()
+            );
             Err(PyTypeError::new_err(err_msg))
         }
     }
@@ -134,7 +140,7 @@ mod tests {
             assert!(err.is_instance_of::<PyTypeError>(py));
             assert_eq!(
                 err.to_string(),
-                "TypeError: failed to convert the value to 'Union[int, float]'"
+                "TypeError: failed to convert the value to 'Union[i32, f32]'"
             );
 
             let obj_i = 42.to_object(py);