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

diff --git a/tests/test_union.py b/tests/test_union.py
index e1866e7..2a2c12f 100644
--- a/tests/test_union.py
+++ b/tests/test_union.py
@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 from pybind11_tests import union_ as m