I think that PEP 11 also doesn't rule out changes in this area. For example, consider the paragraph starting with the sentence "This policy does not disqualify supporting other platforms indirectly. [...]"
Attached is a patch that accomplishes the same with a constant running time for operations on keys by using an array. Allocation of new keys runs in expected constant time by using a randomised allocation pattern.
If we're not big fans of using randomised access, changing this to a linear scan would already be an improvement: keys are only allocated infrequently, but hopefully accessed quite often.