Avoid copy in iteration by using const auto & (#4861)

This change is fixing a Coverity AUTO_CAUSES_COPY issues.

include/pybind11/pybind11.h

@@ -1142,7 +1142,7 @@ protected:
                     }
                     }
                     msg += "kwargs: ";
                     msg += "kwargs: ";
                     bool first = true;
                     bool first = true;
-                    for (auto kwarg : kwargs) {
+                    for (const auto &kwarg : kwargs) {
                         if (first) {
                         if (first) {
                             first = false;
                             first = false;
                         } else {
                         } else {