binman: Remove use of Set()

A new built-in set() is used in both Python 2 and 3 now. Move it to use
that instead of Set().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/bsection.py b/tools/binman/bsection.py
index 0ba542e..be67ff9 100644
--- a/tools/binman/bsection.py
+++ b/tools/binman/bsection.py
@@ -8,7 +8,6 @@
 from __future__ import print_function
 
 from collections import OrderedDict
-from sets import Set
 import sys
 
 import fdt_util
@@ -109,7 +108,7 @@
 
     def GetFdtSet(self):
         """Get the set of device tree files used by this image"""
-        fdt_set = Set()
+        fdt_set = set()
         for entry in self._entries.values():
             fdt_set.update(entry.GetFdtSet())
         return fdt_set