input: Write a function to count the number of unique countries in a list.
  assert unique_countries(["USA", "Canada", "USA", "Mexico", "Canada"]) == 3
  assert unique_countries(["Japan", "Japan", "Japan"]) == 1
  assert unique_countries(["India", "China", "India", "China", "India", "China"]) == 2
output: def unique_countries(country_list):
      return len(set(country_list))