* 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.
