On Python 3.5+, we can make use of scandir that not only list the
content of the directory as an iterator but caches some infomations (for
example, `is_dir()`; this avoid extra stats call to the underlying
filesytem and can be – according to pep 471 – 2x to 20 time faster
especially on NFS filesystem where stats call is expensive.
From a quick this is the only place where scandir would make sens, as
most other places only require the name.
Fixes 1381