NONDETERM should never downgrade to MT19337

[simsong]

Because NONDETERM is secure and MT19337 is not, NONDETERM should never downgrade to MT19337.

Apparently calling .seed() on a NONDETERM random number generator will make it downgrade to MT19337. This behavior should be validated and, if it is true, should be removed.

[oleksandr-pavlyk]

@simsong Method .seed(seed_value, brng=brng) seeds the generator corresponding to brng using the seed_value.

brng defaults to 'MT19937' explaining the behavior.

Your suggestion seems to be change the default to None to be interpreted as MT19937 if called on an instance whose brng is yet unset, but otherwise keeping the brng of the instance.

This is sensible. Will implement.

Sorry I did not see the request any sooner.