assertXMLEqual chokes on document type declaration

[Yuri Kanivetsky]

Prepare Django project:

$ python -m venv env
$ . ./env/bin/activate
$ pip install django
$ django-admin startproject p1
$ cd p1

Create p1/tests.py:

from django.test import TestCase

class MyTestCase(TestCase):
    def test_assert_xml_equal(self):
        xml1 = '''
            <?xml version="1.0" encoding="UTF-8"?>
            <!DOCTYPE root SYSTEM "example.dtd">
            <root />
        '''
        xml2 = '''
            <?xml version="1.0" encoding="UTF-8"?>
            <!DOCTYPE root SYSTEM "example.dtd">
            <root />
        '''
        self.assertXMLEqual(xml1, xml2)

Run the tests:

$ ./manage.py test
Creating test database for alias 'default'...
F
======================================================================
FAIL: test_assert_xml_equal (p1.tests.MyTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yuri/_/1/env/lib/python3.7/site-packages/django/test/testcases.py", line 843, in assertXMLEqual
    result = compare_xml(xml1, xml2)
  File "/home/yuri/_/1/env/lib/python3.7/site-packages/django/test/utils.py", line 598, in compare_xml
    return check_element(want_root, got_root)
AttributeError: 'DocumentType' object has no attribute 'tagName'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/yuri/_/1/p1/p1/tests.py", line 15, in test_assert_xml_equal
    self.assertXMLEqual(xml1, xml2)
  File "/home/yuri/_/1/env/lib/python3.7/site-packages/django/test/testcases.py", line 846, in assertXMLEqual
    self.fail(self._formatMessage(msg, standardMsg))
AssertionError: First or second argument is not valid XML
'DocumentType' object has no attribute 'tagName'

----------------------------------------------------------------------
Ran 1 test in 0.002s

FAILED (failures=1)
Destroying test database for alias 'default'...
System check identified no issues (0 silenced).

That happens because django.utils expects first ​non-comment element to be the ​root element. Which is generally not the case.