input: Write a function to implement a function in Python that takes a Huffman tree and drop all its leaf nodes.
  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((10, (20, None, None), (30, None, None)))==None
  assert dropLeafNodes((5, (6, (7, None, None), (8, None, None)), (9, (10, None, None), (11, None, None))))==(5, (6, None, None), (9, None, None))
  assert dropLeafNodes((5, None, None))==None
  assert dropLeafNodes(None)==None
output: def dropLeafNodes(tree):
      if tree is None:
          return None
      if tree[1] is None and tree[2] is None:
          return None
      left = dropLeafNodes(tree[1])
      right = dropLeafNodes(tree[2])
      return (tree[0], left, right)