Depend on `six` for py3k compatilibity
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Fix flaky file update test on python3.3

Python3.3 introduces a stat cache for os.stat() calls, which can cause
this test to fail.

Solve this by mocking the task.stat method to return a mocked stat object
that returns a different mtime/atime without actually needing to call
os.utime or actually re-stat the file.