In bug 1290765 the ability to install pip requirements.txt files was added to VirtualenvManager. Because CI uses an older version of pip for many tasks, we need to be able to disable pip's hash checking mode (which is only available in versions greater than 8). Once we have an upgraded pip everywhere though, we should always enforce hash checking. It should be noted that any task that has a source checkout available should automatically pick up the in-tree virtualenv (and therefore a modern enough pip).
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