Spead2

- Add :py:const:`spead2.send.GroupMode.SERIAL`.
- Add :py:class:`spead2.send.HeapReferenceList`.
- Speed up C++ unit tests.
- Fix some spurious output in the statistics report from
:program:`spead2_recv.py` (introduced in 3.5.0).
- Fix the help message from :program:`spead2_net_raw` to have the right name
for the program.
- Update to latest version of pybind11.

- Allow a ringbuffer to be stopped only once the last producer has indicated
completion, rather than the first.
- Change :py:class:`.ChunkRingStream` so that stops received from the network
only shut down a shared ringbuffer once all the streams have stopped. A user
call to ``stop`` will still stop the ringbuffer immediately.
- :py:meth:`.ChunkRingbuffer.stop` now returns a boolean to indicate whether
this is the first time the ringbuffer was stopped.

- Add support for :ref:`custom-stats`.
- Change the allocate and ready callbacks on
:cpp:class:`spead2::recv::chunk_stream` to take a pointer to the batch
statistics. This is a **backwards-incompatible change** (keep in mind that
chunking receive is still experimental). Code that uses
:cpp:class:`spead2::recv::chunk_ring_stream` is unaffected.
- Change the design of deleters for
:cpp:class:`spead2::memory_allocator`. Code that calls ``get_user`` or
``get_deleter`` on a pointer allocated by spead2 may now get a ``nullptr``
back. Code that uses a custom memory allocator and that calls these
functions on pointers allocated by that allocator should continue to work.
- Allow a ready callback to be used together with
:cpp:class:`spead2::recv::chunk_ring_stream`, to finish preparation of a
chunk before it pushed to the ringbuffer.
- In Python, avoid copying immediate items when given as 0-d arrays with dtype
``>u8``. This makes it practical to pre-define heaps and later update their
values rather than creating new heap objects.
- Make :py:class:`spead2.send.Stream`, :py:class:`spead2.send.SyncStream` and
:py:class:`spead2.send.asyncio.AsyncStream` available for type annotations.
- Fix an occasional segfault when stopping a
:py:class:`spead2.recv.ChunkRingStream`.

- Add :doc:`recv-chunk`.
- Add missing :py:meth:`spead2.recv.Stream.add_udp_pcap_file_reader` to .pyi file.
- Add :py:meth:`spead2.InprocQueue.add_packet`.
- Prevent conversions from ``None`` to :py:class:`spead2.ThreadPool`.

- :cpp:class:`spead2::recv::mem_reader` now stops the stream gracefully,
allowing incomplete heaps to be flushed.

- Convert :program:`spead2_net_raw` to a C++ file so that it gets the same
compiler flags as everything else.
- Migrate from Travis CI to Github Actions.
- Fix some warnings generated by Clang.
- Fix some test failures with PyPy.