'mock' is built in to python as unittest.mock since python 3.3.
The mock package on pypi is a backport, and not required here.
The test code throughout this repo used a mix of unittest.mock and
the mock package. This makes it consistently unittest.mock now,
and makes the import styles consistent too.
Change-Id: I643247ac4061c095b327a73cba75a1fa67f1c32e
365aed9a21