new windows and OSX build hosts needed

The datalad/git-annex repository is no longer used as an autobuilder of of git-annex. https://github.com/datalad/git-annex/issues/260

Specifically, it was used before for the Windows and OSX autobuilds.

A new build host is needed in order for git-annex releases to be accompanied by Windows and OSX builds.

Note that CI for Windows and OSX is still available through appveyor. But I don't think build artifacts can be extracted from there?

A possible alternative for OSX would be to stop providing builds, and rely on homebrew's builds.

And/or the pipi/uv builds could perhaps be blessed as the official builds. mih assures me those builds will contiue to be from git-annex master.