hacking already pins flake8, so do not add it, especially in conflict
with the hacking version. hacking also pins pycodestyle, so remove that
as well.
The repo is Python 3 now, so update hacking for a python3 version.
Blacklist new failures or fix them.
Convert own hacking checks to new flake8 API, update lower-constraints
for this.
Change-Id: I2928b9d764f61a53a549a9eb850c7809bdbb6f74
30e5a90552