Both W504 (line break after binary operator) and W503 (line break before binary
operator) were implicitly enabled, even though conceptually they are mutually
exclusive.
W504 had over 100 hits, while W503 had none, so let's disable the former.
* E402: we exectute code between imports, like modifying 'sys.path' in the tests
* E731: we assign to lambdas in a lot of places, we may want to consider defining functions in a single line instead (what pep8 recommends)
* 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