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/capabilities/bill.py

@@ -30,6 +30,7 @@ class SubscriptionNotFound(UserError):
     """
     Raised when a subscription is not found.
     """
+
     def __init__(self, msg='Subscription not found'):
         UserError.__init__(self, msg)
 
@@ -38,6 +39,7 @@ class BillNotFound(UserError):
     """
     Raised when a bill is not found.
     """
+
     def __init__(self, msg='Bill not found'):
         UserError.__init__(self, msg)