Fix gdb pretty printers to work with Python 3.

[dblaikie]

Don't know much about Python version differences, but at a glance of the patch & some documentation this seems reasonable.

Glad to see/hear other people are using the pretty printers - how're you finding them?

[ikudrin]

They are really helpful, especially for looking into things like SmallVector, which uses AlignedCharArrayUnion to store items. Personally, I'd prefer debug dumpers for QtCreator, but this variant is more universal.

utils/gdb-scripts/prettyprinters.py

def next(self):
       if self.cur == self.end:
         raise StopIteration
       count = self.count
       self.count = self.count + 1
       cur = self.cur
       self.cur = self.cur + 1
       return '[%d]' % count, cur.dereference()
 
+    __next__ = next
+
   def __init__(self, val):
     self.val = val
 
   def children(self):
     t = self.val.type.template_argument(0).pointer()
     begin = self.val['BeginX'].cast(t)
     end = self.val['EndX'].cast(t)
     return self._iterator(begin, end)
class _iterator:
     def next(self):
       if self.cur == self.end:
         raise StopIteration
       count = self.count
       self.count = self.count + 1
       cur = self.cur
       self.cur = self.cur + 1
       return '[%d]' % count, cur.dereference()
     
+    __next__ = next
+
   def __init__(self, val):
     self.val = val
 
   def children(self):
     data = self.val['Data']
     return self._iterator(data, data + self.val['Length'])
 
   def to_string(self):