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 =============================================================================
```

* [dtype]: add type() method to access type attribute of PyArray_Descr (eq. to dtype.char in Python)

* [dtype] change type() name method to char_() to be compliant with Python numpy interface

* [dtype] fix by pre-commit

* [dtype] Change comments and solutions format for test

* Clarify documentation and move note about dtype.char vs PyArray_Descr::type to a plain, non-doxygen comment

* Fix and extend tests

* Fix the supposedly fixed tests

* Fix the fixed tests again

Co-authored-by: Bertrand MICHEL <bertrand.michel@onera.fr>
Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>

* Splitting out detail/type_caster_base.h from cast.h.
* Include cleanup guided by include-what-you-use 0.12 based on clang version 9.0.1-11.

* Add initial implementation

* Add few more methods

* Add tests

* Fix a typo

* Use std::string constructor which takes size

* Fix implicit sign conversion error

* Add size method and test

* Remove implicit conversion

* Fix bytearray constructors and operator std::string()

* Make implicit bytearray constructor explicit

* Rerun tests

* Add null check

* Rerun tests

* Rerun tests - 2

* Remove NULL check

Co-authored-by: OnlineCop <onlinecop@gmail.com>

Bumps [pypa/gh-action-pypi-publish](https://github.com/pypa/gh-action-pypi-publish) from v1.4.1 to v1.4.2.
- [Release notes](https://github.com/pypa/gh-action-pypi-publish/releases)
- [Commits](https://github.com/pypa/gh-action-pypi-publish/compare/v1.4.1...27b31702a0e7fc50959f5ad993c78deac1bdfc29)

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

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

On Windows, clang-cl does not understand /MP.
```
clang-cl: warning: argument unused during compilation: '/MP' [-Wunused-command-line-argument]
```
with Clang 10.0.0

* Make sure all warnings in pytest get turned into errors

* Suppress DeprecationWarnings in test_int_convert and test_numpy_int_convert

* PyLong_AsLong only shouts "Deprecated!" on Python>=3.8

* Fix remaining warnings on PyPy and CPython 3.10-dev