bpo-22831: Use "with" to avoid possible fd leaks in tools (part 2). (GH-10927)

Tools/scripts/objgraph.py

@@ -180,7 +180,8 @@ def main():
         if filename == '-':
             readinput(sys.stdin)
         else:
-            readinput(open(filename, 'r'))
+            with open(filename) as f:
+                readinput(f)
     #
     warndups()
     #