Replace 'raise StopIteration' with 'return'

With PEP 479, the behaviour of StopIteration is changing. Raising it to
stop a generator is considered incorrect and from Python 3.7 this will
cause a RuntimeError. The PEP recommends using the return statement.

More details: https://www.python.org/dev/peps/pep-0479/#examples-of-breakage

Change-Id: If53f0f4c313a699a4036838ab6035fffa931931b

troveclient/v1/backups.py

@@ -243,7 +243,7 @@ class Backups(base.ManagerWithFind):
                         yield the_item
                     m = the_list[-1].id
                 else:
-                    raise StopIteration()
+                    return
 
         def execution_list_generator():
             yielded = 0
@@ -254,7 +254,7 @@ class Backups(base.ManagerWithFind):
                                             loaded=True)
                     yielded += 1
                 if limit and yielded == limit:
-                    raise StopIteration()
+                    return
 
         return list(execution_list_generator())