hacking 3.0.x is too old. This also removes the note about old behavior
of pip which has been changed in recent versions.
Notes:
- A few errors newly detected are also fixed.
- This also fixes the compatibility with flake8 >= 3.0 because flake8
is also bumped when we bump hacking.
Change-Id: I7fa87ed77bb39945ee5464692b7ef3d25c046127
7787fefa30