Hash auto-randomization is a mechanism to detect when a collision attack is underway and switch to a randomized keying scheme at that point.

This patch is for the 2.7 branch, where hash randomization is not on by default.

Using collided strings from https://github.com/Storyyeller/fnv-collider/tree/master/collided_strings, 10 "attacks" of roughly 50,000 collided strings were launched against this. The unmodified Python had a median insert time of roughly 4.32 seconds and a median retrieve time of roughly 4.40 seconds. With the auto-randomized version of Python, the median insert time was roughly 3.99 seconds and median retrieve time was roughly 3.57 seconds. This is a 7.7% and 18.9% savings, respectively.