input: Write a python function to return a dictionary where each element in the list is a key and its value is the square of the element.
  assert product_of_list([7, 8, 5]) == {7: 49, 8: 64, 5: 25}
  assert product_of_list([1, 2, 3]) == {1: 1, 2: 4, 3: 9}
  assert product_of_list([0, 10, -3]) == {0: 0, 10: 100, -3: 9}
output: def product_of_list(lst):
      return {x: x**2 for x in lst}