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

diff --git a/tests/test_pickling.py b/tests/test_pickling.py
index 5ae05aa..58d67a6 100644
--- a/tests/test_pickling.py
+++ b/tests/test_pickling.py
@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import pytest
 from pybind11_tests import pickling as m