size_report.py does not work with some versions "anytree" package

The report script does not have "any tree" package in dependencies and need some old versions of this package. With version 2.12.1 it falls with this error :

```
File "/mnt/sdb1/nuttxspace/nuttx/./tools/size_report.py", line 815, in
main()
File "/mnt/sdb1/nuttxspace/nuttx/./tools/size_report.py", line 802, in main
root = generate_any_tree(symbol_dict, symsize, common_path_prefix)
File "/mnt/sdb1/nuttxspace/nuttx/./tools/size_report.py", line 559, in generate_any_tree
root = TreeNode("Root", "root")
File "/mnt/sdb1/nuttxspace/nuttx/./tools/size_report.py", line 533, in __init__
self.size = size
AttributeError: can't set attribute 'size'

```

But version 2.8.0 works fine