python: Add double newline after fuction or class (E305).

pycodestyle >= 2.1.0 reports E305 otherwise, and the flake8-check step
fails.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>

python/ovs/fatal_signal.py

@@ -43,6 +43,7 @@ def fork():
             cancel()
 
     _hooks = []
+
 
 _added_hook = False
 _files = {}