input: Write a function to find common words between two strings.
  assert intersection("This is a test string", "This test string is not a drill") == ['This', 'is', 'a', 'test', 'string']
  assert intersection("apple orange banana", "banana apple grape") == ['apple', 'banana']
  assert intersection("hello world", "world peace") == ['world']
output: def intersection(str1, str2):
      set1 = set(str1.split())
      set2 = set(str2.split())
      return sorted(list(set1 & set2))