Switch to the use of `pathlib` where possible

[cidrblock]

trafficstars

as noted in #624

Switch to the use of pathlib where possible away from os.path

[webknjaz]

https://gitlab.com/RoPP/flake8-use-pathlib

[tinvaan]

Fwiw, https://docs.python.org/3/library/pathlib.html#correspondence-to-tools-in-the-os-module