Python 3.9 builds for windows

[h-vetinari]

Merging #138 only built for linux/osx but continues the 3.9-migration. However, all packages depending on scipy and building for windows will necessarily fail without a windows build (from either CF or anaconda). Last time, this was done manually, I believe (cf. #119).

[Anthchirp]

Thanks for creating this ticket to keep track of things. I've retriggered all linked in PRs.