Install broken for Python 3.9

[dbousque]

On macOS 11.2.3, with python 3.9.4 and pip 21.0.1, the native code building part is failing because of the use of some deprecated CPython functions by Cython. Upgrading Cython to 0.29.23 and rebuilding the project should do it. Let me know if you'd like additional information, like error logs.

[christiemj09]

@dbousque Thanks for raising this! Your solution is verified on the branch python-39. I'll make the dependency Cython>=0.29.23 explicit in the CI script and in the documentation.