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
In particular with the right stacklevel IPython will display the warning
if code is directly entered by the user. Without this info it does not.
Use the opportunity to add in the warning since when things are
deprecated. This leads to one less lookup of information for the user.
avoid a couple of dynamic lookup in the core of the parsing loop.
Performance improvement will be minimal, but adding a little many time
can be consequent.
Matthias Bussonnier