input: Write a function to implement a Python function that takes a Huffman tree and drops all its leaf nodes.
  assert dropLeafNodes(None)==None
  assert dropLeafNodes(((None, None), (None, None)))==None
  assert dropLeafNodes(((None, ((None, None), None)), (None, None)))==(None, None)
output: def dropLeafNodes(tree):
      if tree is None:
          return None
      if isinstance(tree, tuple) and len(tree) == 2:
          left, right = tree
          if left is None and right is None:
              return None
          new_left = dropLeafNodes(left)
          new_right = dropLeafNodes(right)
          if new_left is None and new_right is None:
              return None
          return (new_left, new_right)
      return None