Fix lxml for unit tests

[int-y1]

Unit tests are failing because of this error:

  File "/home/runner/work/online-judge/online-judge/judge/migrations/0091_compiler_message_ansi2html.py", line 4, in <module>
    from lxml.html.clean import clean_html
  File "/opt/hostedtoolcache/Python/3.7.17/x64/lib/python3.7/site-packages/lxml/html/clean.py", line 21, in <module>
    ) from None
ImportError: lxml.html.clean module is now a separate project lxml_html_clean.
Install lxml[html_clean] or lxml_html_clean directly.
Error: Process completed with exit code 1.

[codecov-commenter]

Codecov Report

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

Project coverage is 46.76%. Comparing base (bc6aef1) to head (1e51ade).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2326 +/- ## ======================================= Coverage 46.76% 46.76% ======================================= Files 251 251 Lines 13317 13317 ======================================= Hits 6228 6228 Misses 7089 7089 ```

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