Fix DeprecationWarning with newer Python versions

[lcosmin]

Using pytest-logger 1.1.0 with Python 3.12 results in the following DeprecationWarning:

/Users/xxx/.venv/lib/python3.12/site-packages/pytest_logger/plugin.py:310: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
dt = datetime.datetime.utcfromtimestamp(ct)

This commit replaces utcfromtimestamp with fromtimestamp(..., tz=datetime.timezone.utc)

[coveralls]

coverage: 98.662%. remained the same
when pulling 53628f2 on lcosmin:master
into 322b8b9 on aurzenligl:master.

[aurzenligl]

Thanks for the PR! I'm pulling this and releasing as patch.