* Add `npy_format_descriptor<PyObject *>` to enable `py::array_t<PyObject *>` to/from-python conversions.

* resolve clang-tidy warning

* Use existing constructor instead of adding a static method. Thanks @Skylion007 for pointing out.

* Add `format_descriptor<PyObject *>`

Trivial addition, but still in search for a meaningful test.

* Add test_format_descriptor_format

* Ensure the Eigen `type_caster`s do not segfault when loading arrays with dtype=object

* Use `static_assert()` `!std::is_pointer<>` to replace runtime guards.

* Add comments to explain how to check for ref-count bugs. (NO code changes.)

* Make the "Pointer types ... are not supported" message Eigen-specific, as suggested by @Lalaland. Move to new pybind11/eigen/common.h header.

* Change "format_descriptor_format" implementation as suggested by @Lalaland. Additional tests meant to ensure consistency between py::format_descriptor<>, np.array, np.format_parser turn out to be useful only to highlight long-standing inconsistencies.

* resolve clang-tidy warning

* Account for np.float128, np.complex256 not being available on Windows, in a future-proof way.

* Fully address i|q|l ambiguity (hopefully).

* Remove the new `np.format_parser()`-based test, it's much more distracting than useful.

* Use bi.itemsize to disambiguate "l" or "L"

* Use `py::detail::compare_buffer_info<T>::compare()` to validate the `format_descriptor<T>::format()` strings.

* Add `buffer_info::compare<T>` to make `detail::compare_buffer_info<T>::compare` more visible & accessible.

* silence clang-tidy warning

* pytest-compatible access to np.float128, np.complex256

* Revert "pytest-compatible access to np.float128, np.complex256"

This reverts commit e9a289c50fc07199806d14ded644215ab6f03afa.

* Use `sizeof(long double) == sizeof(double)` instead of `std::is_same<>`

* Report skipped `long double` tests.

* Change the name of the new `buffer_info` member function to `item_type_is_equivalent_to`. Add comment defining "equivalent" by example.

* Change `item_type_is_equivalent_to<>()` from `static` function to member function, as suggested by @Lalaland