Fix a long-standing bug in the handling of Python multiple inheritance (#4762)

* Equivalent of 5718e4d080

* Resolve clang-tidy errors.

* Moving test_PPCCInit() first changes the behavior!

* Resolve new Clang dev C++11 errors:

```
The CXX compiler identification is Clang 17.0.0
```

```
pytypes.h:1615:23: error: identifier '_s' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
```

```
cast.h:1380:26: error: identifier '_a' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
```

* Resolve gcc 4.8.5 error:

```
pytypes.h:1615:12: error: missing space between '""' and suffix identifier
```

* Specifically exclude `__clang__`

* Snapshot of debugging code (does NOT pass pre-commit checks).

* Revert "Snapshot of debugging code (does NOT pass pre-commit checks)."

This reverts commit 1d4f9ff263.

* [ci skip] Order Dependence Demo

* Revert "[ci skip] Order Dependence Demo"

This reverts commit d37b5409d4.

* One way to deal with the order dependency issue. This is not the best way, more like a proof of concept.

* Move test_PC() first again.

* Add `all_type_info_add_base_most_derived_first()`, use in `all_type_info_populate()`

* Revert "One way to deal with the order dependency issue. This is not the best way, more like a proof of concept."

This reverts commit eb09c6c1b9.

* clang-tidy fixes (automatic)

* Add `is_redundant_value_and_holder()` and use to avoid forcing `__init__` overrides when they are not needed.

* Streamline implementation and avoid unsafe `reinterpret_cast<instance *>()` introduced with PR #2152

The `reinterpret_cast<instance *>(self)` is unsafe if `__new__` is mocked,
which was actually found in the wild: the mock returned `None` for `self`.
This was inconsequential because `inst` is currently cast straight back to
`PyObject *` to compute `all_type_info()`, which is empty if `self` is not
a pybind11 `instance`, and then `inst` is never dereferenced. However, the
unsafe detour through `instance *` is easily avoided and the updated
implementation is less prone to accidents while debugging or refactoring.

* Fix actual undefined behavior exposed by previous changes.

It turns out the previous commit message is incorrect, the `inst` pointer is actually dereferenced, in the `value_and_holder` ctor here:

f3e0602802/include/pybind11/detail/type_caster_base.h (L262-L263)

```
259     // Main constructor for a found value/holder:
260     value_and_holder(instance *i, const detail::type_info *type, size_t vpos, size_t index)
261         : inst{i}, index{index}, type{type},
262           vh{inst->simple_layout ? inst->simple_value_holder
263                                  : &inst->nonsimple.values_and_holders[vpos]} {}
```

* Add test_mock_new()

* Experiment: specify indirect bases

* Revert "Experiment: specify indirect bases"

This reverts commit 4f90d85f9f.

* Add `all_type_info_check_for_divergence()` and some tests.

* Call `all_type_info_check_for_divergence()` also from `type_caster_generic::load_impl<>`

* Resolve clang-tidy error:

```
include/pybind11/detail/type_caster_base.h:795:21: error: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty,-warnings-as-errors]
                if (matching_bases.size() != 0) {
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~
                    !matching_bases.empty()
```

* Revert "Resolve clang-tidy error:"

This reverts commit df27188dc6.

* Revert "Call `all_type_info_check_for_divergence()` also from `type_caster_generic::load_impl<>`"

This reverts commit 5f5fd6a68e.

* Revert "Add `all_type_info_check_for_divergence()` and some tests."

This reverts commit 0a9599f775.

tests/test_class.py

@@ -1,3 +1,5 @@
+from unittest import mock
+
 import pytest
 
 import env
@@ -203,6 +205,18 @@ def test_inheritance_init(msg):
     assert msg(exc_info.value) == expected
 
 
+@pytest.mark.parametrize(
+    "mock_return_value", [None, (1, 2, 3), m.Pet("Polly", "parrot"), m.Dog("Molly")]
+)
+def test_mock_new(mock_return_value):
+    with mock.patch.object(
+        m.Pet, "__new__", return_value=mock_return_value
+    ) as mock_new:
+        obj = m.Pet("Noname", "Nospecies")
+    assert obj is mock_return_value
+    mock_new.assert_called_once_with(m.Pet, "Noname", "Nospecies")
+
+
 def test_automatic_upcasting():
     assert type(m.return_class_1()).__name__ == "DerivedClass1"
     assert type(m.return_class_2()).__name__ == "DerivedClass2"