Switch tkinter import based on Python version

Rather than try/except, which can cause issues if python-future is
installed.

Closes gh-9822

IPython/terminal/pt_inputhooks/tk.py

@@ -40,10 +40,12 b' will fix it for Tk.)'
 """
 import time
 
+from IPython.utils.py3compat import PY3
+
 import _tkinter
-try:
+if PY3:
     import tkinter
-except ImportError:
+else:
     import Tkinter as tkinter  # Python 2
 
 def inputhook(inputhook_context):