input: Write a function to implement the dropLeafNodes function in Python that takes a Huffman tree and removes all its leaf nodes. The input is a Huffman tree represented by a head node, and the output is the modified tree without its leaf nodes.
  assert dropLeafNodes(None)==None
  assert dropLeafNodes((1, ((2, ((4, None), (5, None))), (3, ((6, None), (7, None))))))==((2, None), (3, None))
  assert dropLeafNodes((1, None))==None
  assert dropLeafNodes((1, ((2, None), (3, None))))==((2, None), (3, None))
output: def dropLeafNodes(head):
      if head is None:
          return None
      if isinstance(head, tuple):
          value, left, right = head[0], head[1], None
          if len(head) > 2:
              right = head[2]
          left = dropLeafNodes(left)
          right = dropLeafNodes(right)
          if left is None and right is None:
              return None
          return (value, left, right)
      return None