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/socks.py

@@ -103,6 +103,7 @@ class ProxyType(object):
     SOCKS4A = 1
     SOCKS5 = 2
 
+
 Proxy = collections.namedtuple('Proxy', (
     'type', 'host', 'port', 'username', 'password', 'remote_dns'))