input: Write a python function to calculate the maximum depth of a binary tree.
  assert calculate_depth(Node(1)) == 1
  assert calculate_depth(Node(1, Node(2), Node(3))) == 2
  assert calculate_depth(Node(1, Node(2, Node(4), Node(5)), Node(3))) == 3
output: class Node:
      def __init__(self, value=0, left=None, right=None):
          self.value = value
          self.left = left
          self.right = right
  
  def calculate_depth(root):
      if not root:
          return 0
      left_depth = calculate_depth(root.left)
      right_depth = calculate_depth(root.right)
      return max(left_depth, right_depth) + 1