In #26839 os.urandom() was made non-blocking and non-exception-raising on Linux. As a result os.urandom() is no longer a CSPRNG under some conditions as it can and will return predictable random values without any sort of warning or error flag. These conditions are (including but not limited to):

* early boot state
* virtualization without host-passthrough, e.g. virtio-rng
* embedded devices without hardware RNG or RTC, e.g. raspberry pi

Please ensure that the documentation properly warns users about these edge cases.

We might also want to add that os.urandom() can block on other platforms, e.g. FreeBSD and OpenBSD.