[MRG] Update macOS version we build on and update Cython pin

[aaronreidsmith]

Description

Cython version 0.29.31 broke our nightly build, so we want to avoid that (we have confirmed that the new 0.29.32 works). Separately macOS 10.15 is deprecated by GitHub so we bump to macos-latest (which is currently macOS 11).

Type of change

Please delete options that are not relevant.

• [X] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

CI/CD passes

Checklist:

• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes