Allow python builtins to be used as callbacks (#1413)

* Allow python builtins to be used as callbacks

* Try to fix pypy segfault

* Add expected fail for PyPy

* Fix typo

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add more info to xfail

* Add env

* Try returning false

* Try removing the move for pypy

* Fix bugs

* Try removing move

* Just keep ignoring for PyPy

* Add back xfail

* Fix ctors

* Revert change of std::move

* Change to skip

* Fix bug and edit comments

* Remove clang-tidy bugprone fix skip bug

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_callbacks.cpp

@@ -156,6 +156,12 @@ TEST_SUBMODULE(callbacks, m) {
         .def(py::init<>())
         .def("triple", [](CppBoundMethodTest &, int val) { return 3 * val; });
 
+    // This checks that builtin functions can be passed as callbacks
+    // rather than throwing RuntimeError due to trying to extract as capsule
+    m.def("test_sum_builtin", [](const std::function<double(py::iterable)> &sum_builtin, const py::iterable &i) {
+      return sum_builtin(i);
+    });
+
     // test async Python callbacks
     using callback_f = std::function<void(int)>;
     m.def("test_async_callback", [](const callback_f &f, const py::list &work) {