Add floordiv to pathlib.Path for strict append

[barneygale]

__abs__() might be useful for exposing a version of absolute() that can eliminate “..” segments at the beginning of a relative path but not elsewhere, as described in this thread:

Making paths absolute in os.path and pathlib

On a POSIX system with symlinks, if os.getcwd() returns "/home/me", what is the absolute version of the relative path "../you/.."? If you answered "/home", you’re in agreement with os.path.abspath(). But if "/home/you" is a symlink to somewhere else in the filesystem, then your result is wrong, as the second ".." component is relative to the symlink target. This behaviour is called “plain wrong” in PEP 428[1]. If you answered "/home/me/../you/..", you’re in agreement with pathlib.Path.absolute…