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


It has not returned the file position after the seek.
(cherry picked from commit 485e715cb1ff92bc9882cd51ec32589f9cb30503)

Co-authored-by: Inada Naoki <songofacandy@gmail.com>
diff --git a/Lib/tempfile.py b/Lib/tempfile.py
index 6287554..5b990e0 100644
--- a/Lib/tempfile.py
+++ b/Lib/tempfile.py
@@ -735,7 +735,7 @@
         return self._file.readlines(*args)
 
     def seek(self, *args):
-        self._file.seek(*args)
+        return self._file.seek(*args)
 
     @property
     def softspace(self):