input: Write a python function to create a Point3D object and return its string representation.
  assert str(Point3D(1, 2, 3)) == 'Point3D is at 1, 2, 3'
  assert str(Point3D(4, 5, 6)) == 'Point3D is at 4, 5, 6'
  assert str(Point3D(0, 0, 0)) == 'Point3D is at 0, 0, 0'
output: def Point3D(x, y, z):
      class Point3D:
          def __init__(self, x, y, z):
              self.x = x
              self.y = y
              self.z = z
  
          def __str__(self):
              return f'Point3D is at {self.x}, {self.y}, {self.z}'
  
      return Point3D(x, y, z)