Fix deprecation warning for datetime.datetime.utcnow

[kfrydel]

Fixes #934

Description

Since Python 3.12, the utcnow function is deprecated - see, https://docs.python.org/3/library/datetime.html#datetime.datetime.utcnow

This commit turns usages of utcnow() into now(UTC).

The feature or problem addressed by this PR

This addresses the issue described in #934

What your changes do and why you chose this solution

The solution is based on the suggestion from official Python docs:
https://docs.python.org/3/library/datetime.html#datetime.datetime.utcnow

Checklist

• Checked that no other issues or pull requests exist for the same issue/change
• Added tests covering the new functionality
• Updated documentation OR the change is too minor to be documented
• Updated CHANGELOG.md OR changes are insignificant

[bdrung]

This merge request does not cover all cases of utcnow and datetime.UTC requires a recent Python version. I addressed those points in #979.

[c00kiemon5ter]

Thank you @kfrydel 👍

As @bdrung mentioned this PR is a good starting point but does not cover all cases.
I am closing this in favour of #979.