The osx builds frequently back up CI, slowing development. Since we don't have any osx specific logic in dask, testing multiple python versions on osx seems unnecessary.

Distributed is planning on cutting out 3.9 and only testing on 3.8 and 3.10 (dask/distributed#6040). We should do the same, or (better yet) drop 2 builds and only test osx on one python version (perhaps 3.9?).