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

youtube_dl/downloader/external.py

@@ -293,6 +293,7 @@ class FFmpegFD(ExternalFD):
 class AVconvFD(FFmpegFD):
     pass
 
+
 _BY_NAME = dict(
     (klass.get_basename(), klass)
     for name, klass in globals().items()