Fix set_next_input on Python 2

IPython/terminal/ptshell.py

@@ -4,7 +4,7 b' from __future__ import print_function'
 import sys
 
 from IPython.core.interactiveshell import InteractiveShell
-from IPython.utils.py3compat import PY3
+from IPython.utils.py3compat import PY3, cast_unicode_py2
 from traitlets import Bool, Unicode, Dict
 
 from prompt_toolkit.completion import Completer, Completion
@@ -177,7 +177,7 b' class PTInteractiveShell(InteractiveShell):'
 
     def pre_prompt(self):
         if self.rl_next_input:
-            self.pt_cli.application.buffer.text = self.rl_next_input
+            self.pt_cli.application.buffer.text = cast_unicode_py2(self.rl_next_input)
             self.rl_next_input = None
 
     def interact(self):