param_source: use random.SystemRandom as random nr source
Python's random module uses a PRNG (Mersenne Twister) which is utterly insecure for key generation - it was so far only used for testing. Replace it with random.SystemRandom(), which draws from /dev/urandom and is suitable for generating cryptographic key material.
validate_val() calls len() to check the value against allow_len, min_len and max_len. len() requires the object to have a __len__() method, which integers do not — calling len() on an int raises TypeError.
Fix this by checking for __len__ first: if present, use len(val) as usual; otherwise fall back to len(str(val)), which gives the number of decimal digits for integer values.
