gh-91768: C API no longer use "const PyObject*" type (#91769)

Py_REFCNT(), Py_TYPE(), Py_SIZE() and Py_IS_TYPE() functions argument
type is now "PyObject*", rather than "const PyObject*".

* Replace also "const PyObject*" with "PyObject*" in functions:

  * _Py_strhex_impl()
  * _Py_strhex_with_sep()
  * _Py_strhex_bytes_with_sep()

* Remove _PyObject_CAST_CONST() and _PyVarObject_CAST_CONST() macros.
* Py_IS_TYPE() can now use Py_TYPE() in its implementation.