Implement :pep:`782`, the :c:type:`PyBytesWriter` API. Add functions:

* :c:func:`PyBytesWriter_Create`
* :c:func:`PyBytesWriter_Discard`
* :c:func:`PyBytesWriter_FinishWithPointer`
* :c:func:`PyBytesWriter_FinishWithSize`
* :c:func:`PyBytesWriter_Finish`
* :c:func:`PyBytesWriter_Format`
* :c:func:`PyBytesWriter_GetData`
* :c:func:`PyBytesWriter_GetSize`
* :c:func:`PyBytesWriter_GrowAndUpdatePointer`
* :c:func:`PyBytesWriter_Grow`
* :c:func:`PyBytesWriter_Resize`
* :c:func:`PyBytesWriter_WriteBytes`

Patch by Victor Stinner.
