Blosc2

* Add `msgpack` as a runtime requirement

* New `pack_array2()` and `unpack_array2()` functions for packing NumPy arrays. Contrarily to `pack_array()` and `unpack_array()` counterparts, the new ones allow for compressing arrays larger than 2 GB in size.

* New `Scunk.to_cframe()` and `blosc2.from_cframe()` methods for serializing/deserialzing `SChunk` instances.

* New `Schunk.get_slice()`, `SChunk.__getitem__()` and `SChunk.__setitem__()` methods for getting/setting slices from/to `SChunk` instances.

* The `compcode` parameter has been renamed to `codec`. A `NameError` exception will be raised when using the old name. Please update your code when you see this exception.

* More doc restructurings. Hopefully, they are more pleasant to read now :-)

* Several leaks fixed. Thanks to Christoph Gohlke.

* Internal C-Blosc2 updated to 2.3.1

* Added a new `blosc2.open(urlpath, mode)` function to be able to open persisted super-chunks.

* Added a new tutorial in notebook format (`examples/tutorial-basics.ipynb`) about the basics of python-blosc2.

* Internal C-Blosc2 updated to 2.2.0

* Internal C-Blosc updated to 2.0.4.

New super-chunk implementation

* New `SChunk` class that allows to create super-chunks.
This includes the capability of storing data in 4
different ways (sparse/contiguous and in memory/on-disk),
as well as storing variable length metalayers.

* Also, during the contruction of a `SChunk` instance,
an arbitrarily large data buffer can be given so that it is
automatically split in chunks and those are appended to the
`SChunk`.

* See `examples/schunk.py` and `examples/vlmeta.py` for some examples.

* Documentation of the new API is here: https://python-blosc2.readthedocs.io

This release is the result of a grant offered by
the Python Software Foundation to Marta Iborra.
A blog entry was written describing the difficulties and relevant
aspects learned during the work:
https://www.blosc.org/posts/python-blosc2-initial-release/

* Release with C-Blosc 2.0.2 sources and binaries.