pep8 blank lines fixes

flake8 --select W391,E302,E301,E304

autopep8 can't fix W391 even though it claims it can.
Fixed using a simple custom script.

weboob/tools/backend.py

@@ -44,6 +44,7 @@ class BackendStorage(object):
     :param storage: storage object
     :type storage: :class:`weboob.tools.storage.IStorage`
     """
+
     def __init__(self, name, storage):
         self.name = name
         self.storage = storage