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Fix of this http://bugs.python.org/issue13703 is broken.
tl;dr: There only 256 different hash functions (compare it to size of _Py_HashSecret prefix and suffix). And whether keys collide or not depends only on the last 8 bits of prefix. 
Problem with current randomization of hash function is following:
Suffix does not influence whether two keys have some hash or not (it is xor-ed after everything). 
Everything except last 8 bits in prefix does not influence it also. Try adding 0x474200 to prefix and see what happens (it will add 0x474200 to resulting hash). 
To make a DoS attack, attacker must do the following:
Generate sets of colliding keys for every 256 possible combinations of last 8 bits. Try each one of these sets - one will have significantly bigger response time, and then repeat this one.