scandir, a better directory iterator and faster os.walk()

scandir() is a generator version of os.listdir() that returns
an iterator over files in a directory, and also exposes the
extra information most OSes provide while iterating files in a
directory (such as type and stat information).

This module also includes a version of os.walk() that uses
scandir() to speed it up significantly.

NOTE: If you're using Python version 3.5+, os.scandir() and the
speed improvements to os.walk() are already available in the
standard library.