flake8: Ignore some error added in pep8 1.6

* 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.cfg

@@ -3,4 +3,4 @@ universal = True
 
 [flake8]
 exclude = youtube_dl/extractor/__init__.py,devscripts/buildserver.py,setup.py,build,.git
-ignore = E501
+ignore = E402,E501,E731