…urn type. (google#5156)

* Transfer diff from pybind11k fork as-is. New tests are still missing.

* Add `PYBIND11_WARNING_DISABLE_MSVC(4127)` into `PYBIND11_OVERRIDE_IMPL` macro.

* Add test_trampoline_with_pyobject_ptr_return()

* Resolve clang-tidy error: use auto when initializing with a template cast to avoid duplicating the type name [modernize-use-auto,-warnings-as-errors]

* Disabled checking refcount when building with PyPy.

* Clang 3.6, 3.7, 3.9 compatibility.

```
/__w/pybind11/pybind11/tests/test_type_caster_pyobject_ptr.cpp:23:13: error: definition of implicit copy constructor for 'WithPyObjectPtrReturn' is deprecated because it has a user-declared destructor [-Werror,-Wdeprecated]
    virtual ~WithPyObjectPtrReturn() = default;
            ^
```

* Minor clean-up of production code changes.

* Add missing `override` (to resolve clang-tidy error).

* Move PYBIND11_WARNING_POP for a better clang-format outcome.

Merge conflicts resolved in:
    include/pybind11/pybind11.h
    tests/test_type_caster_pyobject_ptr.cpp
    tests/test_type_caster_pyobject_ptr.py