fix: accept either quote character in the XML declaration

tests/test_lint_rules.py

@@ -34,6 +34,17 @@ def test_rule_failure_maps_to_its_section(fail_page, name, expected):
     assert (match[0].section, match[0].level) == (section, level)
 
 
+def test_either_quote_character_opens_the_declaration(conforming):
+    """Serialisers differ: lxml writes single quotes, and XML allows them."""
+    page = conforming().replace(
+        b'<?xml version="1.0" encoding="UTF-8"?>',
+        b"<?xml version='1.0' encoding='UTF-8'?>",
+    )
+    report = validate_bytes(page)
+    assert "xml-declaration" not in [f.code for f in report.findings]
+    assert report.conforms
+
+
 def test_entity_declarations_are_refused_before_parsing(fail_page):
     """A page declaring its own entities is rejected without expanding them."""
     report = validate_bytes(fail_page("billion-laughs"))