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

diff --git a/tests/test_builtin_casters.py b/tests/test_builtin_casters.py
index 9142258..9a49ffa 100644
--- a/tests/test_builtin_casters.py
+++ b/tests/test_builtin_casters.py
@@ -1,4 +1,4 @@
-# Python < 3 needs this: coding=utf-8
+# -*- coding: utf-8 -*-
 import pytest
 
 from pybind11_tests import builtin_casters as m