deal with upcoming deprecations in CPython #25912
Description
FAILED lib/matplotlib/tests/test_lines.py::test_set_line_coll_dash_image[svg] - matplotlib.testing.exceptions.ImageComparisonFailure: images not close (RMS 0.412):
FAILED lib/matplotlib/tests/test_backend_svg.py::test_url_tick - DeprecationWarning: datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.now(datetime.UTC).
FAILED lib/matplotlib/tests/test_backend_svg.py::test_svg_default_metadata - DeprecationWarning: datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.now(datetime.UTC).
FAILED lib/matplotlib/tests/test_backend_svg.py::test_svg_clear_default_metadata - DeprecationWarning: datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.now(datetime.UTC).
which was changed by python/cpython#103858
We appear to use this a couple of places in Matplotlib https://github.com/search?q=repo%3Amatplotlib%2Fmatplotlib%20utcfromtimestamp&type=code all of which need to be changed to not-deprecated methods of achieving the same thing.
I am labeling this as "good first issue" because there is no API design and there should be no behavior changes (other than not getting warnings) but "medium" difficulty as datetime and timezones are involved.