Looking into this change, I wonder if there are any platforms
that we support that lack random.SystemRandom.

os.urandom() documentation talks about Linux, Unix-like systems and
Windows. While https://en.wikipedia.org/wiki//dev/random 
explains that we have all major operations systems covered.

As even the salt is a cryptographic purpose, my take on this is that
we should only use random.SystemRandom, which does not need to be seeded
and remove all seeding and fallback code. It should make our code even
simpler.

What do you think?