bpo-34751: improved hash function for tuples

[jdemeyer]

This patch improves the hash code for tuples to avoid the obvious hash collision

hash((3, 3)) == hash((-3, -3))

Pseudo-code of the new hash:

def tuplehash(t):
    h = INITIALVALUE
    for x in t:
        y = hash(x)
        y = mangle(y)
        h = (h ^ y) * MULTIPLIER
    return h + FINALVALUE

def mangle(y):
    return y ^ (2 * y)

This has the structure of a standard FNV-1a hash. The line y = mangle(y) was added to avoid hash collisions for nested tuples and to work around collisions due to x ^ -2 = -x for odd x.

The constants were chosen as follows:

• INITIALVALUE = 3430008: kept from old algorithm
• FINALVALUE = 97531: kept from old algorithm
• MULTIPLIER = 3**41 (truncated to the platform bitsize): a sufficiently big odd number without obvious bit structure. The standard FNV multipliers tended to create more collisions, probably due to the high number of 0 bits.

https://bugs.python.org/issue34751