gh-129813, PEP 782: Use PyBytesWriter in _PyBytes_FromList() (#138837)

Use the new public PyBytesWriter API in:

* _PyBytes_FromHex()
* _PyBytes_FromBuffer()
* _PyBytes_FromList()
* _PyBytes_FromTuple()
* _PyBytes_FromIterator()

Add _PyBytesWriter_ResizeAndUpdatePointer() and
_PyBytesWriter_GetAllocated() helper functions.