pythongh-91810: Fix regression with writing an XML declaration with encoding='unicode'

Suppress writing an XML declaration in open files in ElementTree.write()
with encoding='unicode' and xml_declaration=None.

Author: serhiy-storchaka

Lib/test/test_xml_etree.py

@@ -3760,17 +3760,13 @@ def test_write_to_text_file(self):
             tree.write(f, encoding='unicode')
             self.assertFalse(f.closed)
         with open(TESTFN, 'rb') as f:
-            self.assertEqual(f.read(),  convlinesep(
-                             b'''<?xml version='1.0' encoding='ascii'?>\n'''
-                             b'''<site>&#248;</site>'''))
+            self.assertEqual(f.read(),  b'''<site>&#248;</site>''')
 
         with open(TESTFN, 'w', encoding='ISO-8859-1') as f:
             tree.write(f, encoding='unicode')
             self.assertFalse(f.closed)
         with open(TESTFN, 'rb') as f:
-            self.assertEqual(f.read(),  convlinesep(
-                             b'''<?xml version='1.0' encoding='ISO-8859-1'?>\n'''
-                             b'''<site>\xf8</site>'''))
+            self.assertEqual(f.read(), b'''<site>\xf8</site>''')
 
     def test_write_to_binary_file(self):
         self.addCleanup(os_helper.unlink, TESTFN)

Lib/xml/etree/ElementTree.py

@@ -731,6 +731,8 @@ def write(self, file_or_filename,
         with _get_writer(file_or_filename, encoding) as (write, declared_encoding):
             if method == "xml" and (xml_declaration or
                     (xml_declaration is None and
+                     not (encoding.lower() == "unicode" and
+                          hasattr(file_or_filename, "write")) and
                      declared_encoding.lower() not in ("utf-8", "us-ascii"))):
                 write("<?xml version='1.0' encoding='%s'?>\n" % (
                     declared_encoding,))

Misc/NEWS.d/next/Library/2022-06-02-08-40-58.gh-issue-91810.Gtk44w.rst

@@ -0,0 +1,2 @@
+Suppress writing an XML declaration in open files in ``ElementTree.write()``
+with ``encoding='unicode'`` and ``xml_declaration=None``.