input: Write a python function to calculate the population density of a Country instance.
  assert calculate_density(Country("CountryA", 1000000, 500)) == 2000
  assert calculate_density(Country("CountryB", 500000, 250)) == 2000
  assert calculate_density(Country("CountryC", 1000000, 200)) == 5000
output: def calculate_density(country):
      return country.population / country.area
  
  class Country:
      def __init__(self, name, population, area):
          self.name = name
          self.population = population
          self.area = area