Add pypy to Windows build

[pganssle]

Right now we don't test against pypy or pypy3 on Windows because Appveyor doesn't have it out-of-the-box, but that's caused us to miss some platform-specific bugs (like #448), so we should probably start testing against pypy. Here is a StackOverflow post explaining how to do it.

Ideally, we'd test all PRs against stable pypy and pypy3 (which can be cached), and I'd also like to test master against nightly pypy and pypy3.