input: Write a function to implement a function in Python that takes a Huffman tree and drops all its leaf nodes. Return the modified tree without any leaf nodes. If the head of the tree is None, return None.
  assert dropLeafNodes(None) == None
  assert dropLeafNodes((1, (2, (4, None, None), (5, None, None)), (3, (6, None, None), (7, None, None)))) == (1, (2, None, None), (3, None, None))
  assert dropLeafNodes((1, None, None)) == None
output: def dropLeafNodes(tree):
      if tree is None:
          return None
      if isinstance(tree, tuple) and len(tree) == 3:
          value, left, right = tree
          if left is None and right is None:
              return None
          left = dropLeafNodes(left)
          right = dropLeafNodes(right)
          if left is None and right is None:
              return None
          return (value, left, right)
      return None