Disable mingw warning.

```
cd /d D:\a\pybind11\pybind11\build3\tests && D:\a\_temp\msys64\mingw64\bin\c++.exe -DPYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD_IF_AVAILABLE -DPYBIND11_TEST_BOOST -DPYBIND11_TEST_EIGEN -Dpybind11_tests_EXPORTS @CMakeFiles/pybind11_tests.dir/includes_CXX.rsp -Os -DNDEBUG -fvisibility=hidden -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wundef -Wnon-virtual-dtor -Werror -std=c++17 -MD -MT tests/CMakeFiles/pybind11_tests.dir/test_stl.cpp.obj -MF CMakeFiles\pybind11_tests.dir\test_stl.cpp.obj.d -o CMakeFiles\pybind11_tests.dir\test_stl.cpp.obj -c D:\a\pybind11\pybind11\tests\test_stl.cpp
In file included from D:/a/pybind11/pybind11/include/pybind11/stl.h:32,
                 from D:\a\pybind11\pybind11\tests\test_stl.cpp:10:
In member function 'std::enable_if_t<((__exactly_once<std::variant<_Types>::__accepted_type<_Tp&&, typename std::enable_if<__not_self<_Tp&&>, void>::type> > && is_constructible_v<std::variant<_Types>::__accepted_type<_Tp&&, typename std::enable_if<__not_self<_Tp&&>, void>::type>, _Tp>) && is_assignable_v<std::variant<_Types>::__accepted_type<_Tp&&, typename std::enable_if<__not_self<_Tp&&>, void>::type>&, _Tp>), std::variant<_Types>&> std::variant<_Types>::operator=(_Tp&&) [with _Tp = std::nullptr_t; _Types = {int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}]',
    inlined from 'std::enable_if_t<((__exactly_once<std::variant<_Types>::__accepted_type<_Tp&&, typename std::enable_if<__not_self<_Tp&&>, void>::type> > && is_constructible_v<std::variant<_Types>::__accepted_type<_Tp&&, typename std::enable_if<__not_self<_Tp&&>, void>::type>, _Tp>) && is_assignable_v<std::variant<_Types>::__accepted_type<_Tp&&, typename std::enable_if<__not_self<_Tp&&>, void>::type>&, _Tp>), std::variant<_Types>&> std::variant<_Types>::operator=(_Tp&&) [with _Tp = std::nullptr_t; _Types = {int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}]' at D:/a/_temp/msys64/mingw64/include/c++/12.1.0/variant:1469:2,
    inlined from 'bool pybind11::detail::variant_caster<V<Ts ...> >::load_alternative(pybind11::handle, bool, pybind11::detail::type_list<U, Us ...>) [with U = std::nullptr_t; Us = {}; V = std::variant; Ts = {int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}]' at D:/a/pybind11/pybind11/include/pybind11/stl.h:375:19,
    inlined from 'bool pybind11::detail::variant_caster<V<Ts ...> >::load_alternative(pybind11::handle, bool, pybind11::detail::type_list<U, Us ...>) [with U = double; Us = {std::nullptr_t}; V = std::variant; Ts = {int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}]' at D:/a/pybind11/pybind11/include/pybind11/stl.h:378:32,
    inlined from 'bool pybind11::detail::variant_caster<V<Ts ...> >::load_alternative(pybind11::handle, bool, pybind11::detail::type_list<U, Us ...>) [with U = std::__cxx11::basic_string<char>; Us = {double, std::nullptr_t}; V = std::variant; Ts = {int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}]' at D:/a/pybind11/pybind11/include/pybind11/stl.h:378:32,
    inlined from 'bool pybind11::detail::variant_caster<V<Ts ...> >::load_alternative(pybind11::handle, bool, pybind11::detail::type_list<U, Us ...>) [with U = int; Us = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}; V = std::variant; Ts = {int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}]' at D:/a/pybind11/pybind11/include/pybind11/stl.h:378:32:
D:/a/_temp/msys64/mingw64/include/c++/12.1.0/variant:1475:38: error: 'caster.pybind11::detail::type_caster_odr_guard<std::nullptr_t, pybind11::detail::type_caster<std::nullptr_t, void> >::<unnamed>.pybind11::detail::type_caster<std::nullptr_t, void>::<unnamed>.pybind11::detail::void_caster<std::nullptr_t>::value' may be used uninitialized [-Werror=maybe-uninitialized]
 1475 |             std::get<__index>(*this) = std::forward<_Tp>(__rhs);
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
D:/a/pybind11/pybind11/include/pybind11/stl.h: In function 'bool pybind11::detail::variant_caster<V<Ts ...> >::load_alternative(pybind11::handle, bool, pybind11::detail::type_list<U, Us ...>) [with U = int; Us = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}; V = std::variant; Ts = {int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, std::nullptr_t}]':
D:/a/pybind11/pybind11/include/pybind11/stl.h:373:14: note: 'caster.pybind11::detail::type_caster_odr_guard<std::nullptr_t, pybind11::detail::type_caster<std::nullptr_t, void> >::<unnamed>.pybind11::detail::type_caster<std::nullptr_t, void>::<unnamed>.pybind11::detail::void_caster<std::nullptr_t>::value' was declared here
  373 |         auto caster = make_caster<U>();
      |              ^~~~~~
cc1plus.exe: all warnings being treated as errors
```

include/pybind11/stl.h

@@ -370,12 +370,19 @@ struct variant_caster<V<Ts...>> {
 
     template <typename U, typename... Us>
     bool load_alternative(handle src, bool convert, type_list<U, Us...>) {
+#if defined(__MINGW32__)
+#    pragma GCC diagnostic push
+#    pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
         auto caster = make_caster<U>();
         if (caster.load(src, convert)) {
             value = cast_op<U>(std::move(caster));
             return true;
         }
         return load_alternative(src, convert, type_list<Us...>{});
+#if defined(__MINGW32__)
+#    pragma GCC diagnostic pop
+#endif
     }
 
     bool load_alternative(handle, bool, type_list<>) { return false; }