using UTC timezone singleton instead of datetime.UTC alias

`datetime.UTC` introduced only since 3.11https://docs.python.org/3.11/library/datetime.html#datetime.UTC Fixing it for [3.7 - 3.11) versions ``` Python 3.10.12 (main, May 27 2025, 17:12:29) [GCC 11.4.0] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import datetime >>> datetime.UTC Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: module 'datetime' has no attribute 'UTC' ```