Unify file handling

[xrotwang]

Now that we have py3.5 as baseline, we should unify all file handling to make use of pathlib. I.e. filenames should be handled like encoded strings: Convert to pathlib.Path as early as possible, and pass around pathlib.Path objects internally exclusively.

Also, reading and writing files can then be handled using the read_text and write_text method, etc.

[Anaphory]

Yes! You taught me to love pathlib.Path, this sounds like an excellent plan.

[xrotwang]

@Anaphory and if you still love it, after the rough years of pathlib2 and backports for py2, you'll love it happily ever after.