Add PYBIND11_CONSTINIT, but it does not work for the current use ca…

…ses:

```
g++ -o pybind11/tests/test_numpy_array.os -c -std=c++20 -fPIC -fvisibility=hidden -O0 -g -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wundef -Wnon-virtual-dtor -Wunused-result -Werror -isystem /usr/include/python3.11 -isystem /usr/include/eigen3 -DPYBIND11_STRICT_ASSERTS_CLASS_HOLDER_VS_TYPE_CASTER_MIX -DPYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD_IF_AVAILABLE -DPYBIND11_TEST_BOOST -Ipybind11/include -I/usr/local/google/home/rwgk/forked/pybind11/include -I/usr/local/google/home/rwgk/clone/pybind11/include /usr/local/google/home/rwgk/forked/pybind11/tests/test_numpy_array.cpp
```

```
In file included from /usr/local/google/home/rwgk/forked/pybind11/tests/test_numpy_array.cpp:10:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/numpy.h: In static member function ‘static pybind11::detail::npy_api& pybind11::detail::npy_api::get()’:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/numpy.h:258:82: error: ‘constinit’ variable ‘api_init’ does not have a constant initializer
  258 |         PYBIND11_CONSTINIT static LazyInitializeAtLeastOnceDestroyNever<npy_api> api_init;
      |                                                                                  ^~~~~~~~
```

```
In file included from /usr/local/google/home/rwgk/forked/pybind11/tests/test_numpy_array.cpp:10:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/numpy.h: In static member function ‘static pybind11::object& pybind11::dtype::_dtype_from_pep3118()’:
/usr/local/google/home/rwgk/forked/pybind11/include/pybind11/numpy.h:697:13: error: ‘constinit’ variable ‘imported_obj’ does not have a constant initializer
  697 |             imported_obj;
      |             ^~~~~~~~~~~~
```

include/pybind11/numpy.h

@@ -43,6 +43,14 @@ class array; // Forward declaration
 
 PYBIND11_NAMESPACE_BEGIN(detail)
 
+#ifndef PYBIND11_CONSTINIT
+#    if __cplusplus >= 202002L
+#        define PYBIND11_CONSTINIT constinit
+#    else
+#        define PYBIND11_CONSTINIT
+#    endif
+#endif
+
 // Main author of this class: jbms@
 template <typename T>
 class LazyInitializeAtLeastOnceDestroyNever {
@@ -247,7 +255,7 @@ struct npy_api {
     };
 
     static npy_api &get() {
-        static LazyInitializeAtLeastOnceDestroyNever<npy_api> api_init;
+        PYBIND11_CONSTINIT static LazyInitializeAtLeastOnceDestroyNever<npy_api> api_init;
         return api_init.Get(lookup);
     }
 
@@ -685,7 +693,8 @@ class dtype : public object {
 
 private:
     static object &_dtype_from_pep3118() {
-        static detail::LazyInitializeAtLeastOnceDestroyNever<object> imported_obj;
+        PYBIND11_CONSTINIT static detail::LazyInitializeAtLeastOnceDestroyNever<object>
+            imported_obj;
         return imported_obj.Get([]() {
             return detail::import_numpy_core_submodule("_internal").attr("_dtype_from_pep3118");
         });