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

wx/lib/filebrowsebutton.py

@@ -425,8 +425,7 @@ if __name__ == "__main__":
             self.history={"c:\\temp":1, "c:\\tmp":1, "r:\\temp":1,"z:\\temp":1}
 
         def historyCallBack(self):
-            keys=self.history.keys()
-            keys.sort()
+            keys=sorted(self.history)
             return keys
 
         def OnFileNameChangedHistory (self, event):