chore: replace one last django.utils.timezone.utc with datetime.timezone.utc

[pxpeterxu]

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Motivation

I was trying to deploy DMOJ to a Ubuntu 24.04 LTS server, and for it to work, I had to upgrade to the latest version of Django. I realized that https://github.com/DMOJ/online-judge/commit/6285f972d5e4849c65ad7f21d6830aa469744743 already fixed most cases of django.utils.timezone.utc no longer existing (thanks @kiritofeng!), but there was one instance that was still causing errors.

Changes

• Replace contests.py use of timezone.utc with datetime.timezone.utc too

Testing

Ran:

python3 -Wa manage.py test

on a server with MySQL, etc. installed as recommended in https://docs.dmoj.ca/#/site/installation

[codecov-commenter]

Codecov Report

:x: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 47.73%. Comparing base (fd7fb05) to head (c1fc4d5). :warning: Report is 75 commits behind head on master.

Files with missing lines	Patch %	Lines
judge/views/contests.py	25.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2423      +/-   ##
==========================================
+ Coverage   46.76%   47.73%   +0.96%     
==========================================
  Files         251      260       +9     
  Lines       13317    13746     +429     
==========================================
+ Hits         6228     6561     +333     
- Misses       7089     7185      +96     

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