add python leetcode 993 tag

python-leetcode/README.md

@@ -35,6 +35,7 @@
 21. [ 两数之和 IV - 输入 BST](./solution/tree/leetcode_653_.py)
 22. [左右叶子之和](./solution/tree/leetcode_404_.py)
 23. [路径总和 III](./solution/tree/leetcode_437_.py)
+24. [二叉树的堂兄弟节点](./solution/tree/leetcode_993_.py)