I have indeed seen __get_builtin_constructor fail in practice, in the 
python build in the Ubuntu repository if IIRC. I seem to recall the 
problem was that python was using a version of openssl that didn't 
include all of hash algorithms, probably because a python library or the 
process hosting python also had a dependency on libssl and had it loaded 
already.

But there is also the issue that Python is a language with many 
implementations now, and in fact, it's unlikely that CPython will remain 
the defacto implementation forever. Having a more robust hashlib.py that 
can handle missing algorithm implementations makes a lot of sense (.NET 
for example has no implementation of some of the more esoteric SHA 
hashes.) I remember that in the early days of IronPython, hashlib.py had 
to be completely replaced to be used.