Fully deprecate msgpack-python on pypi

[mbway]

I recently switched my requirements file from using conda to pip/pypi and accidentally missed that the package has a different name (just msgpack) on pypi. In conda-forge the name of the package is still msgpack-python. This led to a different behaviour (unpacking into bytes rather than str by default) which was confusing.

edit: I was confused and hadn't actually checked the pypi page for msgpack-python specifically (which has no description other than to say the package is deprecated), only the msgpack one which refers to msgpack-python being deprecated. It is still confusing when moving from conda-forge to pypi though. Some warning printed during installation or at import might be useful?

[methane]

Actually, that's what I did when I renamed from msgpack-python to msgpack.

See this comment for why I stopped using dummy transition package.
#266 (comment)

[methane]

And upstream issue is not closed yet.
pypa/pip#4961

So pip may not support such transitional package.

[mbway]

would it break things if warnings were added in the meantime until pip is fixed? also with the recent release of pip getting a new resolver (pypa/pip#988) it's possible that the issue is fixed?

[methane]

AFAIK, warning is not shown during pip install.