That directory is temporarily generated when building for PyPi and may be present if something goes wrong with the upload.
@@ -2,5 +2,5 @@
universal = True
[flake8]
-exclude = youtube_dl/extractor/__init__.py,devscripts/buildserver.py,setup.py
+exclude = youtube_dl/extractor/__init__.py,devscripts/buildserver.py,setup.py,build
ignore = E501