fix tricks yaml generation (#965)

This should have failed no matter what since it was attempting to write a string to a file opened as binary.

src/watchdog/watchmedo.py

@@ -376,7 +376,7 @@ def tricks_generate_yaml(args):
     else:
         if not os.path.exists(args.append_to_file):
             content = header + content
-        with open(args.append_to_file, "ab") as output:
+        with open(args.append_to_file, "a", encoding="utf-8") as output:
             output.write(content)