bpo-27319, bpo-31508: Document deprecation in Treeview.selection(). (#3667)

Defer removing old behavior to 3.8.
Document new feature of selection_set() and friends.
diff --git a/Lib/tkinter/ttk.py b/Lib/tkinter/ttk.py
index e1a965e..05c7364 100644
--- a/Lib/tkinter/ttk.py
+++ b/Lib/tkinter/ttk.py
@@ -1404,13 +1404,13 @@
             import warnings
             warnings.warn(
                 "The selop=None argument of selection() is deprecated "
-                "and will be removed in Python 3.7",
+                "and will be removed in Python 3.8",
                 DeprecationWarning, 3)
         elif selop in ('set', 'add', 'remove', 'toggle'):
             import warnings
             warnings.warn(
                 "The selop argument of selection() is deprecated "
-                "and will be removed in Python 3.7, "
+                "and will be removed in Python 3.8, "
                 "use selection_%s() instead" % (selop,),
                 DeprecationWarning, 3)
         else: