closes bpo-42726: gdb libpython: InstanceProxy support for py3 (GH-23912)

On Fedora 31 gdb is using python 3.7.9, calling `proxyval` on an instance with a dictionary fails because of the `dict.iteritems` usage. This PR changes the code to be compatible with py2 and py3.

This changed seemed small enough to not need an issue and news blurb, if one is required please let me know.

Automerge-Triggered-By: GH:benjaminp

Misc/NEWS.d/next/Tools-Demos/2020-12-23-19-42-11.bpo-42726.a5EkTv.rst

@@ -0,0 +1,2 @@
+Fixed Python 3 compatibility issue with gdb/libpython.py handling of attribute
+dictionaries.

Tools/gdb/libpython.py

@@ -468,7 +468,7 @@ class InstanceProxy(object):
     def __repr__(self):
         if isinstance(self.attrdict, dict):
             kwargs = ', '.join(["%s=%r" % (arg, val)
-                                for arg, val in self.attrdict.iteritems()])
+                                for arg, val in self.attrdict.items()])
             return '<%s(%s) at remote 0x%x>' % (self.cl_name,
                                                 kwargs, self.address)
         else: