Fix deprecationwarning for datetime.utcnow

[Matthieu-LAURENT39]

This replaces the call to datetime.utcnow() to datetime.now(timezone.utc).
datetime.utcnow() is deprecated since python 3.12, and as such shows a warning every time save_to_storage() is called.

We use timezone.utc instead of the more recent datetime.utc, since only the former is available in python 3.7

[yobuntu]

should fix : #150