don't wrap comments unnecessarily
[SPARK-2627] use tox.ini to exclude files

No need to exclude files in the call to pep8. The tool references
tox.ini for config information, so we should use that.
[SPARK-2627] undo unnecessary line breaks

“Undo” unnecessary line breaks introduced by accident when I called
autopep8 on the whole Python directory without setting the max line
length to 100. (autopep8 defaults to 79.)