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

diff --git a/tests/test_multiple_inheritance.py b/tests/test_multiple_inheritance.py
index 7a6e4a0..bb602f8 100644
--- a/tests/test_multiple_inheritance.py
+++ b/tests/test_multiple_inheritance.py
@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 import pytest
 from pybind11_tests import ConstructorStats
 from pybind11_tests import multiple_inheritance as m