> How about we generalize SystemRandom so  users can implement
> a custom RNG class wby providing a method rng(amount) -> bytes?

The random module already makes it easy:
"""
Class Random can also be subclassed if you want to use a different basic generator of your own devising: in that case, override the random(), seed(), getstate(), and setstate() methods. Optionally, a new generator can supply a getrandbits() method — this allows randrange() to produce selections over an arbitrarily large range.
"""

> I showed that urandom is just a bit slower than SSL_rand().

"Just a bit" is relative, I read from 2 to 3 times slower, on Linux.

> I don't get it either. urandom is perfectly fine.

Alright, closing then.