[smart_holder] Add gil_scoped_acquire to shared_ptr_trampoline_self_life_support ctor. (#4196)

* Add `gil_scoped_acquire` to `shared_ptr_trampoline_self_life_support` ctor.

* Add test exercising fix & validation that the fix is needed (i.e. this is BROKEN).

test_class_sh_trampoline_shared_ptr_cpp_arg.py::test_std_make_shared_factory[pass_through_shd_ptr] PASSED                            [ 87%]
test_class_sh_trampoline_shared_ptr_cpp_arg.py::test_std_make_shared_factory[pass_through_shd_ptr_release_gil] FAILED                [100%]

```
================================================================= FAILURES =================================================================
______________________________________ test_std_make_shared_factory[pass_through_shd_ptr_release_gil] ______________________________________

pass_through_func = <built-in method pass_through_shd_ptr_release_gil of PyCapsule object at 0x7f1b209707b0>

    @pytest.mark.parametrize(
        "pass_through_func", [m.pass_through_shd_ptr, m.pass_through_shd_ptr_release_gil]
    )
    def test_std_make_shared_factory(pass_through_func):
        class PyChild(m.SpBase):
            def __init__(self):
                super().__init__(0)

        obj = PyChild()
        while True:
>           assert pass_through_func(obj) is obj
E           RuntimeError: NEEDED HERE: gil_scoped_acquire gil;
```

* Put back fix.

Author: rwgk

include/pybind11/detail/smart_holder_type_casters.h

@@ -400,6 +400,7 @@ struct shared_ptr_trampoline_self_life_support {
     PyObject *self;
     explicit shared_ptr_trampoline_self_life_support(instance *inst)
         : self{reinterpret_cast<PyObject *>(inst)} {
+        gil_scoped_acquire gil;
         Py_INCREF(self);
     }
     void operator()(void *) {

tests/test_class_sh_trampoline_shared_ptr_cpp_arg.cpp

@@ -69,6 +69,9 @@ TEST_SUBMODULE(class_sh_trampoline_shared_ptr_cpp_arg, m) {
         .def("has_python_instance", &SpBase::has_python_instance);
 
     m.def("pass_through_shd_ptr", pass_through_shd_ptr);
+    m.def("pass_through_shd_ptr_release_gil",
+          pass_through_shd_ptr,
+          py::call_guard<py::gil_scoped_release>()); // PR #4196
 
     py::classh<SpBaseTester>(m, "SpBaseTester")
         .def(py::init<>())

tests/test_class_sh_trampoline_shared_ptr_cpp_arg.py

@@ -132,12 +132,15 @@ def test_infinite():
         break  # Comment out for manual leak checking (use `top` command).
 
 
-def test_std_make_shared_factory():
+@pytest.mark.parametrize(
+    "pass_through_func", [m.pass_through_shd_ptr, m.pass_through_shd_ptr_release_gil]
+)
+def test_std_make_shared_factory(pass_through_func):
     class PyChild(m.SpBase):
         def __init__(self):
             super().__init__(0)
 
     obj = PyChild()
     while True:
-        assert m.pass_through_shd_ptr(obj) is obj
+        assert pass_through_func(obj) is obj
         break  # Comment out for manual leak checking (use `top` command).