lxml 6.0.0 breaks invalid-HTML tests

With lxml 6.0.0, our unit-tests-pip job fails as follows (https://salsa.debian.org/cjwatson/debusine/-/jobs/7760822):

======================================================================
FAIL: test_invalid_html (debusine.test.tests.test_base.TestCaseTests.test_invalid_html)
Test warnings on invalid HTML.
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Traceback (most recent call last):
  File "/builds/cjwatson/debusine/debusine/test/tests/test_base.py", line 281, in test_invalid_html
    with self.assertRaisesRegex(
AssertionError: AssertionError not raised
======================================================================
FAIL: test_invalid_html_dump_on_error (debusine.test.tests.test_base.TestCaseTests.test_invalid_html_dump_on_error)
Test warnings on invalid HTML with dump_on_error.
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Traceback (most recent call last):
  File "/builds/cjwatson/debusine/debusine/test/tests/test_base.py", line 301, in test_invalid_html_dump_on_error
    with self.assertRaisesRegex(
AssertionError: AssertionError not raised
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This is all in Cython code so I haven't managed to debug it further as yet. It may be best to just pin the older version for now, but this will be a more serious problem once unstable is upgraded to lxml 6.