Replace deprecated datetime.datetime.utcnow() call with datetime.datetime.now(datetime.UTC)

[paulschreiber]

Description

Replace deprecated datetime.datetime.utcnow() call with datetime.datetime.now(datetime.UTC).

From the Python 3.12 readme:

datetime: datetime.datetime’s utcnow() and utcfromtimestamp() are deprecated and will be removed in a future version. Instead, use timezone-aware objects to represent datetimes in UTC: respectively, call now() and fromtimestamp() with the tz parameter set to datetime.UTC. (Contributed by Paul Ganssle in gh-103857.)

Fixes #3088.

[dlm6693]

Thanks for the PR @paulschreiber! I actually have been working on this myself and just opened #3093 so I'm going to close this one. Also please note that the datetime.UTC syntax was only introduced in python 3.11. Since we still support 3.8 - 3.10, we have to use the old syntax datetime.timezone.utc.