input: Write a python function to find the student with the highest score from a dictionary of student scores.
  assert highest_score({'John': 88.0, 'Jane': 91.2, 'Adam': 70.6, 'Josh': 75.1}) == 'Jane'
  assert highest_score({'Paul': 95.5, 'Abby': 87.3, 'Brian': 67.3}) == 'Paul'
  assert highest_score({'Kim': 80.5, 'Mark': 93.2, 'Sarah': 86.7}) == 'Mark'
output: def highest_score(student_scores):
      return max(student_scores, key=student_scores.get)