Check np_random instance and use correct randint alternative

[ishihara-y]

I am not sure when this change was made but in some environment, gym.unwrapped.np_random returns Generator instead of RandomState.

# in case of RandomState
# this line works
gym.unwrapped.np_random.rand_int(...)
# in case of Generator
# rand_int does not exist and we must use integers as an alternative
gym.unwrapped.np_random.integers(...)

This PR will fix this issue and chooses correct function for sampling integers.