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/swfinterp.py

@@ -115,6 +115,8 @@ def _u30(reader):
     res = _read_int(reader)
     assert res & 0xf0000000 == 0
     return res
+
+
 _u32 = _read_int
 
 
@@ -176,6 +178,7 @@ class _Undefined(object):
         return 'undefined'
     __repr__ = __str__
 
+
 undefined = _Undefined()