`tests/test_clean_embed.txt` doctest fails with libxml2 >= 2.15.0

[mgorny]

After upgrading to libxml2 >= 2.15.0 (tested with .0 and .1) and rebuilding lxml:

$ python -m doctest tests/test_clean_embed.txt 
Traceback (most recent call last):
  File "/usr/lib/python3.11/site-packages/lxml/doctestcompare.py", line 95, in check_output
    want_doc = parser(want)
               ^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/lxml/doctestcompare.py", line 68, in html_fromstring
    return etree.fromstring(html, _html_parser)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "src/lxml/etree.pyx", line 3428, in lxml.etree.fromstring
  File "src/lxml/parser.pxi", line 2066, in lxml.etree._parseMemoryDocument
  File "src/lxml/parser.pxi", line 1921, in lxml.etree._parseDoc
  File "src/lxml/parser.pxi", line 1937, in lxml.etree._parseDoc_unicode
  File "src/lxml/parser.pxi", line 1136, in lxml.etree._BaseParser._parseUnicodeDoc
  File "src/lxml/parser.pxi", line 647, in lxml.etree._ParserContext._handleParseResultDoc
  File "src/lxml/parser.pxi", line 765, in lxml.etree._handleParseResult
  File "src/lxml/parser.pxi", line 689, in lxml.etree._raiseParseError
  File "<string>", line 2
lxml.etree.XMLSyntaxError: Unexpected end tag : embed, line 2, column 82

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/usr/lib/python3.11/doctest.py", line 2832, in <module>
    sys.exit(_test())
             ^^^^^^^
  File "/usr/lib/python3.11/doctest.py", line 2824, in _test
    failures, _ = testfile(filename, module_relative=False,
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/doctest.py", line 2115, in testfile
    runner.run(test)
  File "/usr/lib/python3.11/doctest.py", line 1519, in run
    return self.__run(test, compileflags, out)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/doctest.py", line 1372, in __run
    if check(example.want, got, self.optionflags):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/lxml/doctestcompare.py", line 96, in check_output
    except etree.XMLSyntaxError:
           ^^^^^
NameError: name 'etree' is not defined