Fix FlatObsWrapper obs dtype

[pseudo-rnd-thoughts]

Description

Fixes the following problem reported in issue #434 through modifying the dtype of the new obs array to uint8

import gymnasium
from minigrid.wrappers import FlatObsWrapper

env = FlatObsWrapper(gymnasium.make("MiniGrid-Empty-5x5-v0"))
observation, info = env.reset(seed=42)
print(env.observation_space.dtype)
print(observation.dtype)
assert env.observation_space.dtype == observation.dtype