minor test_private_first_base.cpp simplification (after discovering that this can be wrapped with Boost.Python, using boost::noncopyable)

(This demo does NOT involve smart pointers at all, unlike the otherwise similar test_smart_ptr_private_first_base.)

Based on https://godbolt.org/z/4fdjaW by jorgbrown@ (thanks Jorg!).

https://github.com/rwgk/pybind11/tree/xxx_value_ptr_xxx_holder

Systematically exercising casting between shared_ptr<base>, shared_ptr<derived>.

https://github.com/rwgk/pybind11/tree/xxx_value_ptr_xxx_holder

Systematically exercising returning and passing unique_ptr<T>, shared_ptr<T>
with unique_ptr, shared_ptr holder.

Observations:

test_holder_unique_ptr:
  make_unique_pointee  OK
  pass_unique_pointee  BUILD_FAIL (as documented)
  make_shared_pointee  Abort free(): double free detected
  pass_shared_pointee  RuntimeError: Unable to load a custom holder type from a default-holder instance

test_holder_shared_ptr:
  make_unique_pointee  Segmentation fault (#1138)
  pass_unique_pointee  BUILD_FAIL (as documented)
  make_shared_pointee  OK
  pass_shared_pointee  OK

moving prototype code to pybind11/vptr_holder.h, adding type_caster specialization to make the bindings involving unique_ptr passing compile, but load and cast implementations are missing

See also: https://github.com/pybind/pybind11/issues/2583

Does not build with upstream master or
https://github.com/pybind/pybind11/pull/2047, but builds with
https://github.com/RobotLocomotion/pybind11 and almost runs:

```
Running tests in directory "/usr/local/google/home/rwgk/forked/EricCousineau-TRI/pybind11/tests":
================================================================================= test session starts =================================================================================
platform linux -- Python 3.8.5, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: /usr/local/google/home/rwgk/forked/EricCousineau-TRI/pybind11/tests, inifile: pytest.ini
collected 2 items

test_unique_ptr_member.py .F                                                                                                                                                    [100%]

====================================================================================== FAILURES =======================================================================================
_____________________________________________________________________________ test_pointee_and_ptr_owner ______________________________________________________________________________

    def test_pointee_and_ptr_owner():
        obj = m.pointee()
        assert obj.get_int() == 213
        m.ptr_owner(obj)
        with pytest.raises(ValueError) as exc_info:
>           obj.get_int()
E           Failed: DID NOT RAISE <class 'ValueError'>

test_unique_ptr_member.py:17: Failed
============================================================================= 1 failed, 1 passed in 0.06s =============================================================================
```

* Changing pybind11::str to exclusively hold PyUnicodeObject

Factoring out find_registered_python_instance() from type_caster_generic::cast.

* docs: prepare for 2.6.2

* chore: pre-commit autoupdate

Bumps [jwlawson/actions-setup-cmake](https://github.com/jwlawson/actions-setup-cmake) from v1.6 to v1.7.
- [Release notes](https://github.com/jwlawson/actions-setup-cmake/releases)
- [Commits](https://github.com/jwlawson/actions-setup-cmake/compare/v1.6...821902fd0e0da89b13fd3161d358e96bf7550883)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

* Always call PyNumber_Index when casting from Python to a C++ integral type, also pre-3.8

* Fixed on PyPy

* Simplify use of PyNumber_Index, following @rwgk's idea, and ignore warnings in >=3.8

* Reproduce mismatch between pre-3.8 and post-3.8 behavior on __index__ throwing TypeError

* Fix tests on 3.6 <= Python < 3.8

* No, I don't have an uninitialized variable

* Fix use of __index__ on Python 2

* Make types in test_int_convert more ~boring~ descriptive

* Force the builtin module key to be the correct type.

Previously it was always going to be a std::string which converted into
unicode. Python 2 appears to want module keys to be normal str types, so
this was breaking code that expected plain string types in the
builtins.keys() data structure

* Add a simple unit test to ensure all built-in keys are str

* Update the unit test so it will also run on pypy

* Run pre-commit.

Co-authored-by: Jesse Clemens <jesse.clemens@sony.com>

* style: adding clang-format as manual hook

* docs: adding a Clang-Format section. (#2803)

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

* style: avoid using unintialized variables

Tested with cmake --warn-unintialized -S . -B build

* refactor: use function for possibly uninit vars