Upgrade yapf to 0.20.0

Upgrade yapf version to 0.20.0 and reformat Python files.
diff --git a/tools/buildgen/mako_renderer.py b/tools/buildgen/mako_renderer.py
index 0569fa1..acd72bd 100755
--- a/tools/buildgen/mako_renderer.py
+++ b/tools/buildgen/mako_renderer.py
@@ -99,10 +99,10 @@
         elif opt == '-P':
             assert not got_preprocessed_input
             assert json_dict == {}
-            sys.path.insert(
-                0,
-                os.path.abspath(
-                    os.path.join(os.path.dirname(sys.argv[0]), 'plugins')))
+            sys.path.insert(0,
+                            os.path.abspath(
+                                os.path.join(
+                                    os.path.dirname(sys.argv[0]), 'plugins')))
             with open(arg, 'r') as dict_file:
                 dictionary = pickle.load(dict_file)
             got_preprocessed_input = True