mock is unittest.mock since Python 3.3. Piglit requires Python >= 3.6.

From https://github.com/testing-cabal/mock

:
"mock is now part of the Python standard library, available as
unittest.mock in Python 3.3 onwards."

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <!735>