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/state.py b/tools/binman/state.py
index d945e4b..af96786 100644
--- a/tools/binman/state.py
+++ b/tools/binman/state.py
@@ -7,7 +7,6 @@
 
 import hashlib
 import re
-from sets import Set
 
 import os
 import tools
@@ -24,10 +23,10 @@
 use_fake_dtb = False
 
 # Set of all device tree files references by images
-fdt_set = Set()
+fdt_set = set()
 
 # Same as above, but excluding the main one
-fdt_subset = Set()
+fdt_subset = set()
 
 # The DTB which contains the full image information
 main_dtb = None
@@ -136,7 +135,7 @@
     main_dtb = dtb
     fdt_files.clear()
     fdt_files['u-boot.dtb'] = dtb
-    fdt_subset = Set()
+    fdt_subset = set()
     if not use_fake_dtb:
         for image in images.values():
             fdt_subset.update(image.GetFdtSet())