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_download.py

@@ -60,6 +60,7 @@ def _file_md5(fn):
     with open(fn, 'rb') as f:
         return hashlib.md5(f.read()).hexdigest()
 
+
 defs = gettestcases()
 
 
@@ -217,6 +218,7 @@ def generator(test_case):
 
     return test_template
 
+
 # And add them to TestDownload
 for n, test_case in enumerate(defs):
     test_method = generator(test_case)