GitLab

Switches the NaCL Python tests to use the version of mock/pymock
provided by vpython instead of manually importing the version from
//third_party/pymock. This is so that the //third_party version can
eventually be removed.

Also drive-by fixes some unidiomatic-typecheck PyLint warnings that
presubmit complained about.

Bug: 1094489
Change-Id: I886a9eaa9130c8babbefc40281c0de7f984026c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2254583
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Sam Clegg <sbc@chromium.org>
Reviewed-by: Sam Clegg <sbc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780408}