input: Write a python function to create a simple class representing a `Car` with attributes for `make`, `model`, and `year`, and a method to display the car's details.
  ```python
  class Car:
      def __init__(self, make, model, year):
          self.make = make
          self.model = model
          self.year = year
  
      def display_details(self):
          return f"{self.year} {self.make} {self.model}"
  
  assert Car("Toyota", "Camry", 2020).display_details() == "2020 Toyota Camry"
  assert Car("Honda", "Civic", 2018).display_details() == "2018 Honda Civic"
  assert Car("Ford", "Mustang", 2021).display_details() == "2021 Ford Mustang"
output: class Car:
      def __init__(self, make, model, year):
          self.make = make
          self.model = model
          self.year = year
  
      def display_details(self):
          return f"{self.year} {self.make} {self.model}"
  
  assert Car("Toyota", "Camry", 2020).display_details() == "2020 Toyota Camry"
  assert Car("Honda", "Civic", 2018).display_details() == "2018 Honda Civic"
  assert Car("Ford", "Mustang", 2021).display_details() == "2021 Ford Mustang"