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.
- pretty_flake8 tool is removed because it's not commonly used but
needs maintenance effort to keep it in sync with newer releases
Change-Id: I8e91b06199bfbed1d6e9f8d62ddf52fafd60b7f9
7472f515a6