init.py: switch to documented way of extending path

make mypy 0.730 pass cleanly again

swh/__init__.py

-__path__ = __import__('pkgutil').extend_path(__path__, __name__)
+from pkgutil import extend_path
+from typing import Iterable
+
+__path__ = extend_path(__path__, __name__)  # type: Iterable[str]