fix msvc 2019 CI compilation

untests/enum_tests.cpp

@@ -8,12 +8,11 @@
 
 #include "doctest/doctest.hpp"
 
+#include <iostream>
 #include <iterator>
-#include <string_view>
+#include <string>
 #include <type_traits>
 
-using namespace std::string_view_literals;
-
 namespace
 {
     template < typename Enum >