input: Write a function that generates a Python class representing a 3D Vector with attributes x, y, and z, and provides a string representation of the vector.
  class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
  assert str(Vector3D(1, 2, 3))=="(1, 2, 3)"
  assert str(Vector3D(4, 5, 6))=="(4, 5, 6)"
  assert str(Vector3D(0, 0, 0))=="(0, 0, 0)"
output: class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def __str__(self):
          return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"