Spead2

- Add :ref:`spead2_net_raw` tool.
- Eliminate some compiler warnings about unused parameters.
- Update build process to use pypa-build and setuptools_scm.
- Update to pybind11 2.6.2.

- Use ``python3`` instead of ``python`` to invoke Python (so that it works
even on systems where ``python`` is absent or is Python 2).
- Work around a bug that prevented compilation on Boost 1.76.

- Update type annotations to use :class:`numpy.typing.DTypeLike` for dtype
arguments, to prevent false warnings from mypy.

- Add :cpp:func:`spead2::recv::heap::get_payload` to allow the payload
pointer to be retrieved from a complete heap.
- Make the ibverbs sender compatible with `PeerDirect`_.
- Add examples programs showing integration with `gdrcopy`_ and
`PeerDirect`_.
- Always use SFENCE at end of :cpp:func:`memcpy_nontemporal` so that it is
appropriate for use with `gdrcopy`_.
- Fix a memory leak when receiving with ibverbs.

.. _gdrcopy: https://github.com/NVIDIA/gdrcopy
.. _PeerDirect: https://docs.mellanox.com/pages/viewpage.action?pageId=32413288

- Fix installation of header files: some newer headers were not being
installed, breaking builds for C++ projects.

- Fix a use-after-free bug that could cause a crash when freeing a send
stream.
- Improve send performance by eliminating a memory allocation from packet
generation.