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

devscripts/bash-completion.py

@@ -25,5 +25,6 @@ def build_completion(opt_parser):
         filled_template = template.replace("{{flags}}", " ".join(opts_flag))
         f.write(filled_template)
 
+
 parser = youtube_dl.parseOpts()[0]
 build_completion(parser)