Cannot roundtrip Python 3.9 stdlib xml/dom/minidom.py

[bnavigator]

[    2s] + PYTHONPATH=/home/abuild/rpmbuild/BUILDROOT/python-astor-0.8.1-0.x86_64/usr/lib/python3.9/site-packages
[    2s] + PYTHONDONTWRITEBYTECODE=1
[    2s] + pytest-3.9 --ignore=_build.python39 --ignore=_build.python38 -v tests
[    2s] ============================= test session starts ==============================
[    2s] platform linux -- Python 3.9.2, pytest-6.2.2, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3.9
[    2s] cachedir: .pytest_cache
[    2s] rootdir: /home/abuild/rpmbuild/BUILD/astor-0.8.1
[    2s] collecting ... collected 55 items
...
[    9s] =================================== FAILURES ===================================
[    9s] ______________________ RtripTestCase.test_convert_stdlib _______________________
[    9s] 
[    9s] self = <tests.test_rtrip.RtripTestCase testMethod=test_convert_stdlib>
[    9s] 
[    9s]     def test_convert_stdlib(self):
[    9s]         srcdir = os.path.dirname(os.__file__)
[    9s]         result = astor.rtrip.convert(srcdir)
[    9s] >       self.assertEqual(result, [])
[    9s] E       AssertionError: Lists differ: ['/usr/lib64/python3.9/xml/dom/minidom.py'] != []
[    9s] E       
[    9s] E       First list contains 1 additional elements.
[    9s] E       First extra element 0:
[    9s] E       '/usr/lib64/python3.9/xml/dom/minidom.py'
[    9s] E       
[    9s] E       - ['/usr/lib64/python3.9/xml/dom/minidom.py']
[    9s] E       + []
[    9s] 
[    9s] tests/test_rtrip.py:24: AssertionError
[    9s] ------------------------------ Captured log call -------------------------------
[    9s] WARNING  root:rtrip.py:112     calculating dump -- bad
[    9s] WARNING  root:rtrip.py:143 
[    9s] Files failed to round-trip to AST:
[    9s] WARNING  root:rtrip.py:145     /usr/lib64/python3.9/xml/dom/minidom.py
[    9s] =========================== short test summary info ============================
[    9s] FAILED tests/test_rtrip.py::RtripTestCase::test_convert_stdlib - AssertionErr...
[    9s] =================== 1 failed, 52 passed, 2 skipped in 7.26s ====================

Maybe related to #181, because

abuild@skylab:~> diff -u /usr/lib64/python3.[89]/xml/dom/minidom.py
...
@@ -1787,12 +1811,17 @@
             raise xml.dom.NotSupportedErr("cannot import document type nodes")
         return _clone_node(node, deep, self)
 
-    def writexml(self, writer, indent="", addindent="", newl="", encoding=None):
-        if encoding is None:
-            writer.write('<?xml version="1.0" ?>'+newl)
-        else:
-            writer.write('<?xml version="1.0" encoding="%s"?>%s' % (
-                encoding, newl))
+    def writexml(self, writer, indent="", addindent="", newl="", encoding=None,
+                 standalone=None):
+        declarations = []
+
+        if encoding:
+            declarations.append(f'encoding="{encoding}"')
+        if standalone is not None:
+            declarations.append(f'standalone="{"yes" if standalone else "no"}"')
+
+        writer.write(f'<?xml version="1.0" {" ".join(declarations)}?>{newl}')
+
         for node in self.childNodes:
             node.writexml(writer, indent, addindent, newl)