Fix gdb pretty printers to work with Python 3.

[philip.pfaffe]

Summary

Reiterate D23202 for container printers added after the change landed.

[philip.pfaffe]

Ping

[dblaikie]

I don't happen to have a setup to test this - but if it works for you, I'm fine with it :)

utils/gdb-scripts/prettyprinters.py

def __iter__(self):
       return self
 
     def next(self):
       if self.done:
         raise StopIteration
       self.done = True
       return ('value', self.member.dereference())
 
+    __next__ = next
+
   def children(self):
     if not self.value['hasVal']:
       return self._iterator('', True)
     return self._iterator(self.value['storage']['buffer'].address.cast(self.value.type.template_argument(0).pointer()), False)
 
   def to_string(self):
     return 'llvm::Optional is %sinitialized' % ('' if self.value['hasVal'] else 'not ')
 
def next(self):
       if not self.first:
         self.cur = self.cur + 1
         self.advancePastEmptyBuckets()
         self.first = True
       else:
         self.first = False
       return 'x', v
 
+    __next__ = next
+
   def __init__(self, val):
     self.val = val
 
   def children(self):
     t = self.val.type.template_argument(3).pointer()
     begin = self.val['Buckets'].cast(t)
     end = (begin + self.val['NumBuckets']).cast(t)
     return self._iterator(self.val.type.template_argument(2), begin, end)