Use regular ".now" instead of ".utcnow" with UTC zone

[yarikoptic]

It is mandated by starting to receive a DeprecationWarning in python 3.12

DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC)

discovered while addressing some other rot in heudiconv.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.87%. Comparing base (c5fa777) to head (cd4bd5b). Report is 24 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3670 +/- ## ======================================= Coverage 70.86% 70.87% ======================================= Files 1276 1277 +1 Lines 59229 59237 +8 Branches 9812 9813 +1 ======================================= + Hits 41975 41983 +8 Misses 16079 16079 Partials 1175 1175 ```

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[yarikoptic]

eh, it isn't in 3.8 yet, so we have to craft python version specific adapter ...

now fails some other unrelated tests so must be something else (should be string instead of bytes?)

=========================== short test summary info ============================
FAILED algorithms/tests/test_CompCor.py::TestCompCor::test_compcor - ValueError: invalid fmt: b'%.10f'
FAILED algorithms/tests/test_CompCor.py::TestCompCor::test_compcor_variance_threshold_and_metadata - ValueError: invalid fmt: b'%.10f'
FAILED algorithms/tests/test_CompCor.py::TestCompCor::test_tcompcor - ValueError: invalid fmt: b'%.10f'
FAILED algorithms/tests/test_CompCor.py::TestCompCor::test_tcompcor_no_percentile - ValueError: invalid fmt: b'%.10f'
FAILED algorithms/tests/test_CompCor.py::TestCompCor::test_compcor_no_regress_poly - ValueError: invalid fmt: b'%.10f'
FAILED algorithms/tests/test_CompCor.py::TestCompCor::test_tcompcor_asymmetric_dim - ValueError: invalid fmt: b'%.10f'
FAILED algorithms/tests/test_CompCor.py::TestCompCor::test_tcompcor_merge_intersect_masks - ValueError: invalid fmt: b'%.10f'
FAILED algorithms/tests/test_CompCor.py::TestCompCor::test_tcompcor_index_mask - ValueError: invalid fmt: b'%.10f'
FAILED interfaces/cmtk/cmtk.py::nipype.interfaces.cmtk.cmtk.length
FAILED interfaces/mixins/fixheader.py::nipype.interfaces.mixins.fixheader.CopyHeaderInterface
= 10 failed, 2724 passed, 243 skipped, 5 xfailed, 48 warnings in 142.18s (0:02:22) =

[effigies]

We should drop Python 3.8 in any case. It's EOL next week and we won't be releasing before that.