Under python 2.7, the module requires backports.functools_lru_cache>=1.2.1 See https://github.com/crsmithdev/arrow/blob/master/setup.py#L44
This is somehow solved in dev-python/arrow-0.13.1 but not correctly. It now pulls dev-python/backports-functools-lru-cache even if there is no need for python 2.7 which results in huge dependency overhead of all related packages to support also python 2.7. dev-python/arrow-0.12.1 does not have this dependency and works well on python 3.6 only environment.
Fixed with the removal of Python 2 in: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=4f0c44f20d3aa68080fcba0d943a2a933f2db22f