Cleanup casters to release none() to avoid ref counting (pybind#4269)

Author: Skylion007

include/pybind11/cast.h

@@ -248,7 +248,7 @@ struct void_caster {
         return false;
     }
     static handle cast(T, return_value_policy /* policy */, handle /* parent */) {
-        return none().inc_ref();
+        return none().release();
     }
     PYBIND11_TYPE_CASTER(T, const_name("None"));
 };
@@ -291,7 +291,7 @@ class type_caster<void> : public type_caster<void_type> {
         if (ptr) {
             return capsule(ptr).release();
         }
-        return none().inc_ref();
+        return none().release();
     }
 
     template <typename T>
@@ -537,7 +537,7 @@ struct type_caster<CharT, enable_if_t<is_std_char_type<CharT>::value>> {
 
     static handle cast(const CharT *src, return_value_policy policy, handle parent) {
         if (src == nullptr) {
-            return pybind11::none().inc_ref();
+            return pybind11::none().release();
         }
         return StringCaster::cast(StringType(src), policy, parent);
     }

include/pybind11/functional.h

@@ -110,7 +110,7 @@ struct type_caster<std::function<Return(Args...)>> {
     template <typename Func>
     static handle cast(Func &&f_, return_value_policy policy, handle /* parent */) {
         if (!f_) {
-            return none().inc_ref();
+            return none().release();
         }
 
         auto result = f_.template target<function_type>();

include/pybind11/stl.h

@@ -311,7 +311,7 @@ struct optional_caster {
     template <typename T>
     static handle cast(T &&src, return_value_policy policy, handle parent) {
         if (!src) {
-            return none().inc_ref();
+            return none().release();
         }
         if (!std::is_lvalue_reference<T>::value) {
             policy = return_value_policy_override<Value>::policy(policy);