Add a new :c:type:`PyBytesWriter` API to create a Python :class:`bytes` object:

• :c:func:`PyBytesWriter_Create`;
• :c:func:`PyBytesWriter_Finish`;
• :c:func:`PyBytesWriter_Discard`;
• :c:func:`PyBytesWriter_Prepare`;
• :c:func:`PyBytesWriter_WriteBytes`.

Patch by Victor Stinner.