bitbake: COW: Fix StopIteration warning

Fix the warning:

WARNING: lib/bb/data_smart.py:235: DeprecationWarning: generator 'COWDictMeta.iter' raised StopIteration
  for k, v in self.variables.iteritems():

by using return from the generator, not raising StopIteration.

(Bitbake rev: c0af6c81f8d5487ea2cef54a78fd1cb1d0dc6520)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 407d6e07b09123c12c382b4a92107f002c314b05)
Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/COW.py

@@ -150,7 +150,7 @@ class COWDictMeta(COWMeta):
                 yield value
             if type == "items":
                 yield (key, value)
-        raise StopIteration()
+        return
 
     def iterkeys(cls):
         return cls.iter("keys")