input: Write a Python function to create a string representation of a 3D vector.
  assert str(Vector3D(1, 2, 3)) == "(1, 2, 3)"
  assert str(Vector3D(4, 5, 6)) == "(4, 5, 6)"
  assert str(Vector3D(7, 8, 9)) == "(7, 8, 9)"
output: def Vector3D(x, y, z):
      class Vector3D:
          def __init__(self, x, y, z):
              self.x = x
              self.y = y
              self.z = z
  
          def __str__(self):
              return f"({self.x}, {self.y}, {self.z})"
      
      return Vector3D(x, y, z)