bpo-40287: Fix SpooledTemporaryFile.seek() return value (GH-19540)

It has not returned the file position after the seek.
diff --git a/Lib/test/test_tempfile.py b/Lib/test/test_tempfile.py
index f4cba0f..69d5de2 100644
--- a/Lib/test/test_tempfile.py
+++ b/Lib/test/test_tempfile.py
@@ -1018,7 +1018,8 @@
         # Verify writelines with a SpooledTemporaryFile
         f = self.do_create()
         f.writelines((b'x', b'y', b'z'))
-        f.seek(0)
+        pos = f.seek(0)
+        self.assertEqual(pos, 0)
         buf = f.read()
         self.assertEqual(buf, b'xyz')
 
@@ -1036,7 +1037,8 @@
         # when that occurs
         f = self.do_create(max_size=30)
         self.assertFalse(f._rolled)
-        f.seek(100, 0)
+        pos = f.seek(100, 0)
+        self.assertEqual(pos, 100)
         self.assertFalse(f._rolled)
         f.write(b'x')
         self.assertTrue(f._rolled)