update type hinting to follow python 3.11 syntax by using | instead…

… of `typing.Union` and referring to standard collections directly rather than `typing.Collection`

dreem/io/visualize.py

@@ -4,7 +4,6 @@
 from copy import deepcopy
 from tqdm import tqdm
 from omegaconf import DictConfig
-from typing import Union
 
 import seaborn as sns
 import imageio