Replace open(filename, 'rU') by open(filename, 'r')

The U flag is no more used (but still accepted for backward compatibility).
diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index 461518c..ddde01c 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -82,7 +82,7 @@
     def test_fileobj_iter(self):
         self.tar.extract("ustar/regtype", TEMPDIR)
         tarinfo = self.tar.getmember("ustar/regtype")
-        with open(os.path.join(TEMPDIR, "ustar/regtype"), "rU") as fobj1:
+        with open(os.path.join(TEMPDIR, "ustar/regtype"), "r") as fobj1:
             lines1 = fobj1.readlines()
         fobj2 = self.tar.extractfile(tarinfo)
         try: