workaround for missing 'assert_called_once' method

this method was introduced in Python 3.6, but calling it still
works (i.e. it doesn't cause the test to fail) on Python 3.3/3.4

gallery_dl/version.py

@@ -6,4 +6,4 @@
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
-__version__ = "0.9.1"
+__version__ = "1.0.0-dev"

test/test_cookies.py

@@ -69,7 +69,7 @@ class TestCookiejar(unittest.TestCase):
         with mock.patch.object(log, "warning") as mock_warning:
             cookies = extractor.find("test:").session.cookies
             self.assertEqual(len(cookies), 0)
-            mock_warning.assert_called_once()
+            self.assertEqual(mock_warning.call_count, 1)
             self.assertEqual(mock_warning.call_args[0][0], "cookies: %s")
             self.assertIsInstance(mock_warning.call_args[0][1], exc)
 

test/test_extractors.py

@@ -58,7 +58,7 @@ skip = [
     "exhentai", "kissmanga", "mangafox", "dynastyscans", "nijie",
     "archivedmoe", "archiveofsins", "thebarchive",
     # temporary issues
-
+    "twitter",
 ]
 # enable selective testing for direct calls
 if __name__ == '__main__' and len(sys.argv) > 1: