Fix Python lint and unit tests

One of the unit tests added in
https://webrtc-codereview.appspot.com/50079004/ is failing
on Windows since os.sep is a backslash on Windows.
The code is based on the contents of the DEPS file rather than
the filesystem, so the right thing is to use '/' instead of os.sep.

The PyLint blacklist also didn't work on Windows, causing it
to process a massive list of files during presubmit.

I also added a bunch of new entries to speed up lint execution on
all platforms.

TESTED=Ran the presubmit with this CL on Windows and Linux.
R=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/52029004

Cr-Commit-Position: refs/heads/master@{#9353}
2 files changed