Fix deprecation warning for datetime.datetime.utcnow

[kfrydel]

This fixes #934.
utcnow function, starting from Python 3.12 is deprecated: https://docs.python.org/3/library/datetime.html#datetime.datetime.utcnow

Thus, this commit turns usages of utcnow() into now(UTC).

Description

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.