Update coding style after pycodestyle 2.1.0

In pycodestyle 2.1.0, E305 was introduced, which requires two blank
lines after top level declarations, too.

See https://github.com/PyCQA/pycodestyle/issues/400

See also #10689; thanks @stepshal for first mentioning this issue and
initial patches

test/test_write_annotations.py

@@ -24,6 +24,7 @@ class YoutubeDL(youtube_dl.YoutubeDL):
         super(YoutubeDL, self).__init__(*args, **kwargs)
         self.to_stderr = self.to_screen
 
+
 params = get_params({
     'writeannotations': True,
     'skip_download': True,
@@ -74,5 +75,6 @@ class TestAnnotations(unittest.TestCase):
     def tearDown(self):
         try_rm(ANNOTATIONS_FILE)
 
+
 if __name__ == '__main__':
     unittest.main()