Fix clang-tidy

Author: virtuald

tests/test_exceptions.cpp

@@ -340,7 +340,7 @@ TEST_SUBMODULE(exceptions, m) {
         return py::str("UNEXPECTED");
     });
 
-    m.def("test_hidden_error", [](py::function fn) {
+    m.def("test_hidden_error", [](const py::function &fn) {
         // function returns none instead of int, should give a useful error message
         fn().cast<int>();
     });