using UTC timezone singleton instead of datetime.UTC alias

[gyts]

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
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>>> import datetime
>>> datetime.UTC
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'datetime' has no attribute 'UTC'

[lonvia]

Thanks.

We really need more tests there.