fix: try to get MSVC support

Author: henryiii

include/pybind11/pybind11.h

@@ -1979,15 +1979,15 @@ struct iterator_access {
 template <typename Iterator, typename ResultType = decltype(((*std::declval<Iterator>()).first)) >
 struct iterator_key_access {
     using result_type = decltype(((*std::declval<Iterator>()).first));
-    result_type operator()(Iterator &it) const {
+    typename std::add_lvalue_reference<result_type>::type operator()(Iterator &it) const {
         return (*it).first;
     }
 };
 
 template <typename Iterator, typename ResultType = decltype(((*std::declval<Iterator>()).second))>
 struct iterator_value_access {
     using result_type = decltype(((*std::declval<Iterator>()).second));
-    result_type operator()(Iterator &it) const {
+    typename std::add_lvalue_reference<result_type>::type operator()(Iterator &it) const {
         return (*it).second;
     }
 };