Python3: define a __next__() method for VerboseIteratorWrapper

In Python 3, __next__() has replaced next(). Also, call the next() function
rather than the next() method.

Closes-Bug: #1281866
Change-Id: I92b44508c9c875f16ad89ef8410d2c38092ab23d

glanceclient/common/progressbar.py

@@ -15,6 +15,8 @@
 
 import sys
 
+import six
+
 
 class _ProgressBarBase(object):
     """
@@ -78,7 +80,7 @@ class VerboseIteratorWrapper(_ProgressBarBase):
 
     def next(self):
         try:
-            data = self._wrapped.next()
+            data = six.next(self._wrapped)
             # NOTE(mouad): Assuming that data is a string b/c otherwise calling
             # len function will not make any sense.
             self._display_progress_bar(len(data))
@@ -89,3 +91,6 @@ class VerboseIteratorWrapper(_ProgressBarBase):
                 # output.
                 sys.stdout.write('\n')
             raise
+
+    # In Python 3, __next__() has replaced next().
+    __next__ = next