PKG: Set max pin for Cython
We know that pandas doesn't work with Cython 3.0
(https://github.com/pandas-dev/pandas/issues/34213,
https://github.com/pandas-dev/pandas/issues/34014)

This sets the maximum supported version of Cython in our pyproject.toml
to ensure that pandas 1.1.0 can continue to be built from source without
Cython pre-installed after Cython 3.0 is released.
cool