Recent setuptools releases require Python >= 3.9.

https://setuptools.pypa.io/en/latest/history.html#v75-4-0

Maybe it makes sense to delay the SPDX license stuff for a while?

Nitpick: it is Cython 3.1.1, not 3.11 (commit comment, PR title).

The SPDX license stuff requires setuptools >= 77.0.0.

https://setuptools.pypa.io/en/latest/history.html#v77-0-0

So it is either SPDX license stuff or python 3.8 support.

Recent setuptools emits warning about license = {text="Apache 2.0"} when building from source.
But I decided to ignore it for now. Next msgpack version (v1.1.1) will ship with Cython 3.1.1 and Python 3.8 support.