Fixes issue #1556

Replaced XXX.keys() calls with idiomatic python 3 calls that are compatible with python 2 such as eg:
 * replaced "y = xxx.keys()" or "y = list(xxx.keys())" with just "y = list(xxx)"
 * replaced "sorted(xxx.keys())" or "sorted(list(xxx.keys()))" with just "sorted(xxx)"
 * replaced "if not A in B.keys():" with "if A not in B:"
 * replaced "for A in B.keys():"  with "for A in B:"

See also https://python-future.org/compatible_idioms.html
https://python-future.org/compatible_idioms.html#dict-keys-values-items-as-a-list

samples/floatcanvas/GroupDemo.py

@@ -71,7 +71,7 @@ class DrawFrame(wx.Frame):
         tb = NC.ToolBar
 #        tb.AddSeparator()
 
-        for Group in self.Groups.keys():
+        for Group in self.Groups:
             Button = wx.Button(tb, wx.ID_ANY, "Hide/Show%s"%Group)
             tb.AddControl(Button)
             print(Group)