sixish: skip mock if unavailable

Since Python 2 doesn't include mock in the stdlib, not everyone has it.
While for unittests we make people install it, for normal hook usage,
we don't want that.  So don't require it to be imported all the time.
When running unittests, it'll still result in a missing failure.

Bug: 28298984
Test: unittests & upload still pass w/out mock installed
Change-Id: Ia9e66775df7d2150537acd0a6383b48d3303b491