fix: func_handle for rule of two (#3169)

* Fix func_handle for rule of two

* Apply reviewer suggestion

Author: Skylion007

include/pybind11/functional.h

@@ -70,9 +70,11 @@ struct type_caster<std::function<Return(Args...)>> {
         struct func_handle {
             function f;
             func_handle(function &&f_) noexcept : f(std::move(f_)) {}
-            func_handle(const func_handle& f_) {
+            func_handle(const func_handle &f_) { operator=(f_); }
+            func_handle &operator=(const func_handle &f_) {
                 gil_scoped_acquire acq;
                 f = f_.f;
+                return *this;
             }
             ~func_handle() {
                 gil_scoped_acquire acq;