input: Write a function to create a Python class that represents a 3D Vector. The class should have an initializer to set values for x, y, and z, and a __str__ method to return a string representation of the vector in the format "(x, y, z)".
  
  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(0, 0, 0)) == "(0, 0, 0)"
  assert str(Vector3D(-1, 5, -3)) == "(-1, 5, -3)"
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) + ")"