init.py: switch to documented way of extending path
make mypy 0.730 pass cleanly again
In addition to make mypy pass again, this is also the documented way of using extend_path, so it's better/nicer to switch to it anyway.
Once this is accepted for this module, I will batch apply this change to all other init.py around.
Migrated from D2036 (view on Phabricator)