input: Write a python function to calculate the grade level based on the average of a list of grades.
  assert get_grade_level([90, 92, 95]) == 'A'
  assert get_grade_level([80, 85, 88]) == 'B'
  assert get_grade_level([70, 75, 78]) == 'C'
  assert get_grade_level([60, 65, 68]) == 'D'
  assert get_grade_level([50, 55, 58]) == 'F'
output: def get_grade_level(grades):
      average = sum(grades) / len(grades)
      if average >= 90:
          return 'A'
      elif average >= 80:
          return 'B'
      elif average >= 70:
          return 'C'
      elif average >= 60:
          return 'D'
      else:
          return 'F'