devel/py-flake8-future-annotations: new port

Verifies python 3.7+ files use from future import annotations if a
type is used in the module that can be rewritten using PEP 563.

Pairs well with pyupgrade with the --py37-plus flag or higher, since
pyupgrade only replaces type annotations with the PEP 563 rules if
from future import annotations is present.