fix: make gil.h self-contained for PyPy/GraalPy and silence -Wredundant-decls

gil.h evaluated PYBIND11_SIMPLE_GIL_MANAGEMENT before including common.h,
which defines it on PyPy/GraalPy. Every existing TU included common.h
first through pybind11.h, so this only surfaced when src/type_caster_base.cpp
reached gil.h directly. Also suppress GCC -Wredundant-decls for the
isinstance_generic declaration duplicated in pytypes.h.

Assisted-by: ClaudeCode:claude-fable-5

include/pybind11/detail/type_caster_base.h

@@ -219,7 +219,11 @@ public:
 // (get_value_and_holder, allocate_layout, and deallocate_layout are declared inside
 // struct instance in detail/common.h; definitions are in type_caster_base-inl.h.)
 
+PYBIND11_WARNING_PUSH
+PYBIND11_WARNING_DISABLE_GCC("-Wredundant-decls")
+// also forward-declared in pytypes.h
 bool isinstance_generic(handle obj, const std::type_info &tp);
+PYBIND11_WARNING_POP
 
 handle get_object_handle(const void *ptr, const detail::type_info *type);
 

include/pybind11/gil.h

@@ -9,9 +9,12 @@
 
 #pragma once
 
+// common.h must come first: on PyPy/GraalPy it defines PYBIND11_SIMPLE_GIL_MANAGEMENT,
+// which selects the branch below.
+#include "detail/common.h"
+
 #if defined(PYBIND11_SIMPLE_GIL_MANAGEMENT)
 
-#    include "detail/common.h"
 #    include "gil_simple.h"
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
@@ -23,7 +26,6 @@ PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
 
 #else
 
-#    include "detail/common.h"
 #    include "detail/internals.h"
 
 #    include <cassert>