chore: Remove deprecated datetime method calls

[schloerke]

The method "utcfromtimestamp" in class "datetime" is deprecated
Use timezone-aware objects to represent datetimes in UTC; e.g. by calling .fromtimestamp(datetime.UTC) (reportDeprecated)

[schloerke]

Merging as lints are fixed

[wch]

I think the changelog should mention that this is a potentially breaking change, since the returned object is different, and the string formatting is different.

now = datetime.utcnow().timestamp()

y = datetime.utcfromtimestamp(now)
y
#> datetime.datetime(2024, 2, 21, 5, 37, 58, 761000)
print(y)
#> 2024-02-21 05:37:58.761000

x = datetime.fromtimestamp(now, timezone.utc)
x
#> datetime.datetime(2024, 2, 21, 5, 37, 58, 761000, tzinfo=datetime.timezone.utc)
print(x)
# 2024-02-21 05:37:58.761000+00:00