gh-155742: Use PyBytesWriter in _Py_strhex_impl() #157734

Replace soft deprecated PyBytes_FromStringAndSize() with PyBytesWriter.

Replace also PyBytes_FromStringAndSize(NULL, 0) with Py_GetConstant(Py_CONSTANT_EMPTY_BYTES) in other functions.

Replace also PyBytes_FromStringAndSize(NULL, 0) with PyBytes_FromStringAndSize("", 0) in _Py_GetConstant_Init().

• Issue: gh-155742
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