Add __next__() methods to utils iterators for py3

On Python 3, next(obj) calls obj.__next__(), not obj.next(). Add an
alias from __next__() to next() to be compatible with Python 2 and
Python 3.

Change-Id: Ida104d3bd7cdba557e523f18df43d56847060054

swift/common/utils.py

@@ -1065,6 +1065,7 @@ class RateLimitedIterator(object):
                 self.running_time = ratelimit_sleep(self.running_time,
                                                     self.elements_per_second)
         return next_value
+    __next__ = next
 
 
 class GreenthreadSafeIterator(object):
@@ -1088,6 +1089,7 @@ class GreenthreadSafeIterator(object):
     def next(self):
         with self.semaphore:
             return next(self.unsafe_iter)
+    __next__ = next
 
 
 class NullLogger(object):
@@ -1127,6 +1129,7 @@ class LoggerFileObject(object):
 
     def next(self):
         raise IOError(errno.EBADF, 'Bad file descriptor')
+    __next__ = next
 
     def read(self, size=-1):
         raise IOError(errno.EBADF, 'Bad file descriptor')
@@ -2302,6 +2305,7 @@ class GreenAsyncPile(object):
             rv = self._responses.get()
         self._pending -= 1
         return rv
+    __next__ = next
 
 
 class ModifiedParseResult(ParseResult):

test/unit/common/test_utils.py

@@ -3778,6 +3778,7 @@ class UnsafeXrange(object):
                 return val
         finally:
             self.concurrent_calls -= 1
+    __next__ = next
 
 
 class TestAffinityKeyFunction(unittest.TestCase):