ci: GHA basic format & pre-commit (#2309)

diff --git a/tests/test_async.py b/tests/test_async.py
index e1c959d..e9292c9 100644
--- a/tests/test_async.py
+++ b/tests/test_async.py
@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import asyncio
 import pytest
 from pybind11_tests import async_module as m