use assert[Not]IsInstance where appropriate
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
index 09b113b..afeb616 100644
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -133,7 +133,7 @@
             fp = open(test_support.TESTFN)
             try:
                 fd = posix.dup(fp.fileno())
-                self.assertTrue(isinstance(fd, int))
+                self.assertIsInstance(fd, int)
                 os.close(fd)
             finally:
                 fp.close()
@@ -273,7 +273,7 @@
     def test_umask(self):
         if hasattr(posix, 'umask'):
             old_mask = posix.umask(0)
-            self.assertTrue(isinstance(old_mask, int))
+            self.assertIsInstance(old_mask, int)
             posix.umask(old_mask)
 
     def test_strerror(self):