Make build matrix parametrizable from calling workflow.

The CI.yml reusable workflow builds on a matrix of OS and Python versions, which are hardcoded. This means that we had to create a copy, CI_win.yml, with a different hardcoded OS matrix in order to run a Windows build for one consuming repo. Also, if we wanted to change the Python support window for a given repo without changing it for others, we'd need to make another copy with a different Python matrix.

It would be great to figure out how simply pass in the matrix lists from the consuming workflow. We could hardcode a default if we wanted.