input: def create_3d_vector_class() -> type:
      """ Create and return a Python class which represents a 3D Vector with an
      initializer to set the x, y, and z coordinates, and a __str__ method to
      return the vector in the format "(x, y, z)".
      >>> vec = create_3d_vector_class()()
      >>> vec.x = 1
      >>> vec.y = 2
      >>> vec.z = 3
      >>> str(vec)
      '(1, 2, 3)'
      """
      class Vector3D:
          def __init__(self, x=0, y=0, z=0):
              self.x = x
              self.y = y
              self.z = z
          
          def __str__(self):
              return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
      return Vector3D
output:     class Vector3D:
          def __init__(self, x=0, y=0, z=0):
              self.x = x
              self.y = y
              self.z = z
          
          def __str__(self):
              return "(" + str(self.x) + ", " + str(self.y) + ", " + str(self.z) + ")"
      return Vector3D