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

diff --git a/tests/conftest.py b/tests/conftest.py
index 58ebeca..d317c49 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 """pytest configuration
 
 Extends output capture as needed by pybind11: ignore constructors, optional unordered lines.