Increase the level of PEP8 compliance

[madarche]

In the .travis.yml file, there are many exclusions of PEP8 conformance points: --ignore=E501,W604,W601,E721,E402,E711,E713,E741

Those exclusions have no reason apart from the lack of time when this repository was setup in GitHub.

Since the pep8 tool is at the moment the only safeguard, it would be better to use it to its full extent.

[pimzand]

I suppose this is even worse for the python3-mb branch. I would be guilty of that.
Maybe we should revert 377bebc and c4cfd91 if possible

[zzarne]

Reverting them would be of little help as most of the statements there were too long before those commits. Line continuation was ugly before, and I'm to blame mostly.
I think we should just fix what we have, little by little. And wherever possible make the code more readable.
This is a valid issue and we should keep it open until fixed.