bpo-42519: Replace PyMem_MALLOC() with PyMem_Malloc() (GH-23586)

No longer use deprecated aliases to functions:

* Replace PyMem_MALLOC() with PyMem_Malloc()
* Replace PyMem_REALLOC() with PyMem_Realloc()
* Replace PyMem_FREE() with PyMem_Free()
* Replace PyMem_Del() with PyMem_Free()
* Replace PyMem_DEL() with PyMem_Free()

Modify also the PyMem_DEL() macro to use directly PyMem_Free().