Fix deprecation warnings for datetime.utcnow()

[tomasr8]

This shows up when running the tests on 3.12:

~dev/babel/tests/test_dates.py:757: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to rep
resent datetimes in UTC: datetime.datetime.now(datetime.UTC).                                                                                                                                 
    assert dates.format_datetime(datetime.utcnow(), locale="ja_JP@mod")

I think there's no harm in using plain datetime.now() for these tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.22%. Comparing base (cf7d223) to head (0969ad5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1119   +/-   ##
=======================================
  Coverage   91.22%   91.22%           
=======================================
  Files          27       27           
  Lines        4561     4561           
=======================================
  Hits         4161     4161           
  Misses        400      400           

Flag	Coverage Δ
macos-12-3.10	90.02% <ø> (ø)
macos-12-3.11	89.95% <ø> (ø)
macos-12-3.12	90.13% <ø> (ø)
macos-12-3.13-dev	89.65% <ø> (ø)
macos-12-3.8	89.95% <ø> (ø)
macos-12-3.9	89.95% <ø> (ø)
macos-12-pypy3.10	90.02% <ø> (ø)
ubuntu-22.04-3.10	90.04% <ø> (ø)
ubuntu-22.04-3.11	89.98% <ø> (ø)
ubuntu-22.04-3.12	90.15% <ø> (ø)
ubuntu-22.04-3.13-dev	89.67% <ø> (ø)
ubuntu-22.04-3.8	89.97% <ø> (ø)
ubuntu-22.04-3.9	89.97% <ø> (ø)
ubuntu-22.04-pypy3.10	90.04% <ø> (ø)
windows-2022-3.10	90.16% <ø> (ø)
windows-2022-3.11	90.10% <ø> (ø)
windows-2022-3.12	90.27% <ø> (ø)
windows-2022-3.13-dev	89.79% <ø> (ø)
windows-2022-3.8	90.09% <ø> (ø)
windows-2022-3.9	90.09% <ø> (ø)
windows-2022-pypy3.10	90.16% <ø> (ø)

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