Introduce PYBIND11_REINTERPRET_CAST_VOID_PTR_IF_NOT_PYPY to resolve…

… PyPy build errors:

```
In file included from /Users/runner/work/pybind11/pybind11/tests/test_stl.cpp:18:
/Users/runner/work/pybind11/pybind11/include/pybind11/stl/filesystem.h:75:17: error: no matching function for call to 'PyPyUnicode_FSConverter'
            if (PyUnicode_FSConverter(buf, reinterpret_cast<void *>(&native)) != 0) {
                ^~~~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:969:31: note: expanded from macro 'PyUnicode_FSConverter'
                              ^~~~~~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:970:17: note: candidate function not viable: cannot convert argument of incomplete type 'void *' to 'struct _object **' for 2nd argument
PyAPI_FUNC(int) PyUnicode_FSConverter(struct _object *arg0, struct _object **arg1);
                ^
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:969:31: note: expanded from macro 'PyUnicode_FSConverter'
                              ^
In file included from /Users/runner/work/pybind11/pybind11/tests/test_stl.cpp:18:
/Users/runner/work/pybind11/pybind11/include/pybind11/stl/filesystem.h:83:17: error: no matching function for call to 'PyPyUnicode_FSDecoder'
            if (PyUnicode_FSDecoder(buf, reinterpret_cast<void *>(&native)) != 0) {
                ^~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:971:29: note: expanded from macro 'PyUnicode_FSDecoder'
                            ^~~~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:972:17: note: candidate function not viable: cannot convert argument of incomplete type 'void *' to 'struct _object **' for 2nd argument
PyAPI_FUNC(int) PyUnicode_FSDecoder(struct _object *arg0, struct _object **arg1);
                ^
/Users/runner/hostedtoolcache/PyPy/3.10.14/x64/include/pypy3.10/pypy_decl.h:971:29: note: expanded from macro 'PyUnicode_FSDecoder'
                            ^
```

include/pybind11/stl/filesystem.h

@@ -33,6 +33,13 @@
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
 
+#ifdef PYPY_VERSION
+#    define PYBIND11_REINTERPRET_CAST_VOID_PTR_IF_NOT_PYPY(...)
+#else
+#    define PYBIND11_REINTERPRET_CAST_VOID_PTR_IF_NOT_PYPY(...)                                   \
+        reinterpret_cast<void *>(__VA_ARGS__)
+#endif
+
 #if defined(PYBIND11_HAS_FILESYSTEM) || defined(PYBIND11_HAS_EXPERIMENTAL_FILESYSTEM)
 template <typename T>
 struct path_caster {
@@ -72,15 +79,17 @@ struct path_caster {
         }
         PyObject *native = nullptr;
         if constexpr (std::is_same_v<typename T::value_type, char>) {
-            if (PyUnicode_FSConverter(buf, reinterpret_cast<void *>(&native)) != 0) {
+            if (PyUnicode_FSConverter(buf, PYBIND11_REINTERPRET_CAST_VOID_PTR_IF_NOT_PYPY(&native))
+                != 0) {
                 if (auto *c_str = PyBytes_AsString(native)) {
                     // AsString returns a pointer to the internal buffer, which
                     // must not be free'd.
                     value = c_str;
                 }
             }
         } else if constexpr (std::is_same_v<typename T::value_type, wchar_t>) {
-            if (PyUnicode_FSDecoder(buf, reinterpret_cast<void *>(&native)) != 0) {
+            if (PyUnicode_FSDecoder(buf, PYBIND11_REINTERPRET_CAST_VOID_PTR_IF_NOT_PYPY(&native))
+                != 0) {
                 if (auto *c_str = PyUnicode_AsWideCharString(native, nullptr)) {
                     // AsWideCharString returns a new string that must be free'd.
                     value = c_str; // Copies the string.