input: Write a function to remove all duplicate elements from a list of integers. The function should return the modified list with all duplicates removed.
  assert remove_duplicates([1, 2, 3, 2, 4, 5, 6, 3])==[1, 2, 3, 4, 5, 6]
  assert remove_duplicates([4, 4, 4, 4])==[4]
  assert remove_duplicates([1, 2, 3, 4, 5])==[1, 2, 3, 4, 5]
  assert remove_duplicates([])==[]
output: def remove_duplicates(lst):
      return list(dict.fromkeys(lst))