Use flake8 instead of pyflakes and pep8

It combines both tools
Move flake8 configuration to setup.cfg

It will be used when calling flake8 from any directory in the project
flake8: Add more ignored files

* setup.py: the '__version__' variable is not defined in the script, it is loadded from youtube_dl/version.py
* devscripts/buildserver.py: Produces a lot of messages
[mit] Modernize
Improve code style