fix: Render `py::function` as `Callable` (#4829)

* fix: Render `py::function` as `Callable`

* style: pre-commit fixes

---------

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

include/pybind11/cast.h

@@ -898,6 +898,10 @@ struct handle_type_name<float_> {
     static constexpr auto name = const_name("float");
     static constexpr auto name = const_name("float");
 };
 };
 template <>
 template <>
+struct handle_type_name<function> {
+    static constexpr auto name = const_name("Callable");
+};
+template <>
 struct handle_type_name<none> {
 struct handle_type_name<none> {
     static constexpr auto name = const_name("None");
     static constexpr auto name = const_name("None");
 };
 };

tests/test_callbacks.py

@@ -216,3 +216,10 @@ def test_custom_func():
 def test_custom_func2():
 def test_custom_func2():
     assert m.custom_function2(3) == 27
     assert m.custom_function2(3) == 27
     assert m.roundtrip(m.custom_function2)(3) == 27
     assert m.roundtrip(m.custom_function2)(3) == 27
+
+
+def test_callback_docstring():
+    assert (
+        m.test_tuple_unpacking.__doc__.strip()
+        == "test_tuple_unpacking(arg0: Callable) -> object"
+    )