CI run against latest deps is failing

[github-actions]

See https://github.com/element-hq/synapse/actions/runs/16312708593

[anoadragon453]

#18577 fixed the incompatibility with Twisted 25.5.0.

[anoadragon453]

This now just needs an update to the workflow due to poetry dropping --no-dev: https://github.com/element-hq/synapse/actions/runs/15848500213/job/44676069920

[anoadragon453]

After #18617, the tests.media.test_html_preview.OpenGraphFromHtmlTestCase.test_no_tree is now failing upon upgrading Python 3.12 -> 3.13:

===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/work/code/synapse/tests/media/test_html_preview.py", line 329, in test_no_tree
    self.assertIsNone(tree)
  File "/nix/store/51g7phka1hxinnfpan2hclx86xy1qji2-python3-3.13.3-env/lib/python3.13/unittest/case.py", line 1303, in assertIsNone
    self.fail(self._formatMessage(msg, standardMsg))
twisted.trial.unittest.FailTest: <Element html at 0x7f51049477c0> is not None

tests.media.test_html_preview.OpenGraphFromHtmlTestCase.test_no_tree
-------------------------------------------------------------------------------

likely due to something in python stdlib updating revolving around HTML encoding.

[anoadragon453]

It's due to lxml updating from 5.4.0 -> 6.0.0. test_no_tree checks whether lxml.etrees fromstring method returns None for an HTML body consisting of a single null byte b"\x00".

lxml 5.4.0 would would return None, whereas 6.0.0 returns an empty HTML tree.

#18622 should fix that.

[anoadragon453]

Tests are now passing! https://github.com/element-hq/synapse/actions/runs/16295532252

There's some flaky Complement tests, but they don't appear related to dependency updates.