Simplify various spots where: str() is called on something

that already is a string or the existence of the str class
is checked or a check is done for str twice. These all stem
from the initial unicode->str replacement.

Lib/test/test_iter.py

@@ -526,7 +526,7 @@ class TestCase(unittest.TestCase):
         # and pass that on to unicode.join().
         try:
             got = " - ".join(OhPhooey(f))
-            self.assertEqual(got, str("a\n - b\n - fooled you! - c\n"))
+            self.assertEqual(got, "a\n - b\n - fooled you! - c\n")
         finally:
             f.close()
             try: