Create a PetStore class, a Dog class, and Cat class. Once instantiated, a PetStore should be able to take in and give out pets. The store's method for receiving pets should be called receive. This method should take a single pet object (either a Dog or Cat instance) and should add it to the store's inventory of pets. The store's method for giving out a pet should be called sell and can take no arguments. The method returns a pet when called.