clang-tidy readability-qualified-auto (#3702)

* Adding readability-qualified-auto to .clang-tidy

Ported from @henryiii's https://github.com/pybind/pybind11/pull/3683/commits/287527f705c8badee8ba9f6278d691fe0450ae43

* fix: support Python < 3.6

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

tests/test_callbacks.cpp

@@ -148,7 +148,7 @@ TEST_SUBMODULE(callbacks, m) {
         py::arg("expect_none") = false);
     m.def("test_dummy_function", [](const std::function<int(int)> &f) -> std::string {
         using fn_type = int (*)(int);
-        auto result = f.target<fn_type>();
+        const auto *result = f.target<fn_type>();
         if (!result) {
             auto r = f(1);
             return "can't convert to function pointer: eval(1) = " + std::to_string(r);