input: Write a python function to find the student with the highest mark in the dictionary.
  assert find_top_student({"John": 90, "Levi": 85, "Jill": 95, "Sara": 75}) == "Jill"
  assert find_top_student({"Tom": 78, "Lucy": 88, "Anna": 91, "Mark": 85}) == "Anna"
  assert find_top_student({"Alice": 68, "Bob": 73, "Charlie": 73, "Daisy": 68}) == "Bob"
output: def find_top_student(students):
      return max(students, key=students.get)