Implemented __reduce__ method for the ImportError exception (RustPython#4973)

Author: LucaSforza

vm/src/exceptions.rs

@@ -1182,6 +1182,20 @@ pub(super) mod types {
             )?;
             Ok(())
         }
+        #[pymethod(magic)]
+        fn reduce(exc: PyBaseExceptionRef, vm: &VirtualMachine) -> PyTupleRef {
+            let obj = exc.as_object().to_owned();
+            let mut result: Vec<PyObjectRef> = vec![
+                obj.class().to_owned().into(),
+                vm.new_tuple((exc.get_arg(0).unwrap(),)).into(),
+            ];
+
+            if let Some(dict) = obj.dict().filter(|x| !x.is_empty()) {
+                result.push(dict.into());
+            }
+
+            result.into_pytuple(vm)
+        }
     }
 
     #[pyexception(name, base = "PyImportError", ctx = "module_not_found_error", impl)]