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/test/test_ttk/test_widgets.py b/Lib/tkinter/test/test_ttk/test_widgets.py
index 26766a8..ab0db28 100644
--- a/Lib/tkinter/test/test_ttk/test_widgets.py
+++ b/Lib/tkinter/test/test_ttk/test_widgets.py
@@ -1556,7 +1556,7 @@
         self.tv.selection_toggle((c1, c3))
         self.assertEqual(self.tv.selection(), (c3, item2))
 
-        if sys.version_info >= (3, 7):
+        if sys.version_info >= (3, 8):
             import warnings
             warnings.warn(
                 'Deprecated API of Treeview.selection() should be removed')
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: