Return a string of n random bytes suitable for cryptographic use.
This function returns random bytes from an OS-specific
randomness source. The returned data should be unpredictable enough for
cryptographic applications, though its exact quality depends on the OS
implementation. On a UNIX-like system this will query /dev/urandom, and
on Windows it will use CryptGenRandom. If a randomness source is not
found, NotImplementedError will be raised.
New in version 2.4.
