input: Write a function to implement in Python that takes a Huffman tree and drop all its leaf nodes. The function should return the modified tree.
  assert dropLeafNodes(((2, (4, 5)), (3, (6, 7))))==((2, 3), (None, None))
  assert dropLeafNodes((2, (4, 5)))==(2, (None, None))
  assert dropLeafNodes(4)==None
  assert dropLeafNodes(((2, 3), (4, 5)))==(((None, None), (None, None)), ((None, None), (None, None)))
  assert dropLeafNodes(None)==None
output: def dropLeafNodes(tree):
      if tree is None:
          return None
      if isinstance(tree, int):
          return None
      if isinstance(tree[0], int) and isinstance(tree[1], int):
          return (None, None)
      return (dropLeafNodes(tree[0]), dropLeafNodes(tree[1]))