Issue #16120: Use |yield from| in stdlib.

Patch by Berker Peksag.
diff --git a/Lib/unittest/loader.py b/Lib/unittest/loader.py
index 541884e..a5ac737 100644
--- a/Lib/unittest/loader.py
+++ b/Lib/unittest/loader.py
@@ -291,8 +291,7 @@
                         # tests loaded from package file
                         yield tests
                     # recurse into the package
-                    for test in self._find_tests(full_path, pattern):
-                        yield test
+                    yield from self._find_tests(full_path, pattern)
                 else:
                     try:
                         yield load_tests(self, tests, pattern)