input: Write a python function to create a generator for the first n square numbers.
  assert list(sqr_gen(5)) == [0, 1, 4, 9, 16]
  assert list(sqr_gen(3)) == [0, 1, 4]
  assert list(sqr_gen(0)) == []
output: def sqr_gen(n):
      for i in range(n):
          yield i * i