fix: copy strings on Python 2

Author: henryiii

include/pybind11/embed.h

@@ -144,7 +144,9 @@ inline void set_interpreter_argv(int argc, const char* const* argv, bool add_pro
     auto pysys_argv = widened_argv.get();
 #else
     // python 2.x
-    char** pysys_argv = const_cast<char**>safe_argv;
+    std::vector<std::string> strings{safe_argv, safe_argv+argc};
+    std::vector<char*> char_strings{std::begin(strings), std::end(strings)};
+    char** pysys_argv = char_strings.data();
 #endif
 
     PySys_SetArgvEx(argc, pysys_argv, static_cast<int>(add_program_dir_to_path));