init.py: switch to documented way of extending path

make mypy 0.730 pass cleanly again

swh/__init__.py

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