Use a wrapper around xml.etree.ElementTree.fromstring in python 2.x (#7178)

Attributes aren't unicode objects, so they couldn't be directly used in info_dict fields (for example '--write-description' doesn't work with bytes).

test/test_compat.py

@@ -13,8 +13,10 @@ sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 from youtube_dl.utils import get_filesystem_encoding
 from youtube_dl.compat import (
     compat_getenv,
+    compat_etree_fromstring,
     compat_expanduser,
     compat_shlex_split,
+    compat_str,
     compat_urllib_parse_unquote,
     compat_urllib_parse_unquote_plus,
 )
@@ -71,5 +73,10 @@ class TestCompat(unittest.TestCase):
     def test_compat_shlex_split(self):
         self.assertEqual(compat_shlex_split('-option "one two"'), ['-option', 'one two'])
 
+    def test_compat_etree_fromstring(self):
+        xml = '<el foo="bar"></el>'
+        doc = compat_etree_fromstring(xml.encode('utf-8'))
+        self.assertTrue(isinstance(doc.attrib['foo'], compat_str))
+
 if __name__ == '__main__':
     unittest.main()