refactor: get rid of compat.py35path

shutil and os modules in python 3.5 doesn't accept pathlib.Path (yet).

compat.py35path() wrapper was used to mark code which requires extra str() conversion on Python <= 3.5.

Introduce new apkg.util.shutil35 wrapper module for selected shutil and os functions in order to address this without polluting code with redundant py35path/str conversions.

apkg.compat was removed in favor of new shutil35 which can be easily removed in the future once Python 3.5 support is dropped.

Fixes: #36 (closed)