flake8: Ignore E741 'ambiguous variable name'

Most of violating codes are reverse-engineered JavaScripts. IMO it's
better to keep original (obfuscated) names.

[skip ci]

setup.cfg

@@ -3,4 +3,4 @@ universal = True
 
 [flake8]
 exclude = youtube_dl/extractor/__init__.py,devscripts/buildserver.py,devscripts/lazy_load_template.py,devscripts/make_issue_template.py,setup.py,build,.git
-ignore = E402,E501,E731
+ignore = E402,E501,E731,E741