Disable __VA_OPT__(, ) __VA_ARGS__ usage for MSVC (it is unclear to…

… rwgk why it does not work).

This is the beginning of the error message:

```
         D:\a\pybind11\pybind11\tests\test_smart_ptr.cpp(285,1): error C2143: syntax error: missing ')' before ',' [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
         D:\a\pybind11\pybind11\tests\test_smart_ptr.cpp(285,1): error C2059: syntax error: ')' [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
```

include/pybind11/cast.h

@@ -881,7 +881,7 @@ struct always_construct_holder {
         : public type_caster_holder<type, __VA_ARGS__> {};                                        \
     }                                                                                             \
     PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
-#if defined(PYBIND11_CPP20)
+#if defined(PYBIND11_CPP20) && !defined(_MSC_VER)
 #    define PYBIND11_DECLARE_HOLDER_TYPE(type, holder_type, ...)                                  \
         PYBIND11_DECLARE_HOLDER_TYPE_PART1(type, holder_type)                                     \
         void __VA_OPT__(, ) __VA_ARGS__ PYBIND11_DECLARE_HOLDER_TYPE_PART2(type, holder_type)