Resolve helpful clang-tidy errors.

```
/usr/bin/cmake -E __run_co_compile --tidy="/usr/bin/clang-tidy;--use-color;--warnings-as-errors=*;--extra-arg-before=--driver-mode=g++" --source=/__w/pybind11/pybind11/tests/test_class.cpp -- /usr/bin/c++ -DPYBIND11_ENABLE_TYPE_CASTER_ODR_GUARD_IF_AVAILABLE -DPYBIND11_TEST_EIGEN -Dpybind11_tests_EXPORTS -I/__w/pybind11/pybind11/include -isystem /usr/include/python3.9 -isystem /__w/pybind11/pybind11/build/_deps/eigen-src -Os -DNDEBUG -fPIC -fvisibility=hidden -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wundef -Wnon-virtual-dtor -flto=thin -std=c++17 -o CMakeFiles/pybind11_tests.dir/test_class.cpp.o -c /__w/pybind11/pybind11/tests/test_class.cpp
/__w/pybind11/pybind11/include/pybind11/detail/smart_holder_poc.h:114:5: error: single-argument constructors must be marked explicit to avoid unintentional implicit conversions [google-explicit-constructor,-warnings-as-errors]
    custom_deleter(D &&deleter) : deleter{std::move(deleter)} {}
    ^
    explicit
/__w/pybind11/pybind11/include/pybind11/detail/smart_holder_poc.h:120:76: error: forwarding reference passed to std::move(), which may unexpectedly cause lvalues to be moved; use std::forward() instead [bugprone-move-forwarding-reference,-warnings-as-errors]
    return guarded_delete(std::function<void(void *)>(custom_deleter<T, D>(std::move(uqp_del))),
                                                                           ^~~~~~~~~
                                                                           std::forward<D>
```

include/pybind11/detail/smart_holder_poc.h

@@ -111,14 +111,14 @@ guarded_delete make_guarded_builtin_delete(bool armed_flag) {
 template <typename T, typename D>
 struct custom_deleter {
     D deleter;
-    custom_deleter(D &&deleter) : deleter{std::move(deleter)} {}
+    explicit custom_deleter(D &&deleter) : deleter{std::move(deleter)} {}
     void operator()(void *raw_ptr) { deleter(static_cast<T *>(raw_ptr)); }
 };
 
 template <typename T, typename D>
 guarded_delete make_guarded_custom_deleter(D &&uqp_del, bool armed_flag) {
-    return guarded_delete(std::function<void(void *)>(custom_deleter<T, D>(std::move(uqp_del))),
-                          armed_flag);
+    return guarded_delete(
+        std::function<void(void *)>(custom_deleter<T, D>(std::forward<D>(uqp_del))), armed_flag);
 }
 
 template <typename T>