treesum: FIX: Proper error handling. Many errors are not detected at scandir times but then symlinks is to be followed or the content of a file is to be read. Note also that the size of a file can be available (because its directory is readable) but the content of the file is not.

# Pure-Python-Build
[build-system]
requires = ["setuptools>=43.0", "wheel>=0.33"]
build-backend = "setuptools.build_meta"