gh-155742: Use PyMem_Malloc() in memoryview.hex() - #158582

The function was modified in Python 3.15 to replace PyBytes_FromStringAndSize(NULL, size) with PyBytesWriter. I noticed later that no bytes object is needed, so a simple PyMem_Malloc() is enough.

Change similar to PR gh-157587 which replaced PyBytes_FromStringAndSize(NULL, size) with PyMem_Malloc(size).

Note: for simple views (ex: memoryview(bytes)), MV_C_CONTIGUOUS() code path is taken: no memory is copied, _Py_strhex_with_sep() is called directly on the view buffer.

cc @methane

Merged. Thanks for the review, I'm always more confident when a core dev can double check my change ;-)

I was a little bit worried that PyBuffer_ToContiguous() can modify src->len, but this function treats src as constant, so we are fine :-)