Dtype kind vs char (#2864)

* [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>