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

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

* chore: get PyPy 3.7 wheels using NumPy 1.20

* Add Valgrind suppressions after NumPy update

Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>

* Demonstrate issue with weakref constructor overloads

* Fix weakref constructor to convert on being passed a non-weakref object

* Improve on nonlocal-scoped variable in test_weakref

* Keep backwards-compatibility by introducing PYBIND11_OBJECT_CVT_DEFAULT macro

* Simplify test_weakref

Adding holder_caster `typename SFINAE = void` hooks to help work around the current lack of smart-pointer interoperability (#2833)

* Adding move_only_holder_caster `typename SFINAE = void` to enable external specializations.

* Adding SFINAE hook also to copyable_holder_caster, for uniformity, with comment to explain the purpose.

* When determining if a shared_ptr already exists, use a test on the weak_ptr instead of a try/catch block.

* When determining if a shared_ptr already exists, use a test on the weak_ptr instead of a try/catch block.

* weak_from_this is only available in C++17 and later

* Switch to use feature flag instead of C++ version flag.

* Add Microsoft-specific check.

* Avoid undefined preprocessor macro warning treated as error.

* Simplify shared_from_this in init_holder

* Include <version> in detail/common.h (~stolen~ borrowed from @bstaletic's #2816)

* Move class_::get_shared_from_this to detail::try_get_shared_from_this

* Simplify try_get_shared_from_this by using weak_ptr::lock()

Co-authored-by: Yannick Jadoul <yannick.jadoul@belgacom.net>

* Changing pybind11::str to exclusively hold PyUnicodeObject

Factoring out find_registered_python_instance() from type_caster_generic::cast.