Configure CI testing on Windows

[dgw]

Travis is pretty great at what it does, but it only offers Unix-like environments (Linux & macOS). Since Appveyor is also available free for open-source projects, and runs its CI jobs on Windows, it seems like a no-brainer to run tests there as well.

At the moment, known problems on Windows include #1142 (pyenchant is not available for 64-bit Windows) and Appveyor builds are not necessarily expected to succeed. But with PRs like #1164 in progress, actual Windows support is not that far off. And since testing best-practice is to write a failing test first, then implement the functionality it checks, I'll be working on getting Appveyor configured properly for Sopel in the near future. (Status updates will be posted here as relevant.)

[dgw]

Here's the updated plan for this:

1. Defer to dev cycle for Sopel 8 (version 7 needs to release soon, and this would be a distraction)
2. We'll probably use either Travis' new Windows environment or switch all CI to GitHub Actions, eliminating in either case the need to maintain more than one CI service config
3. Reduced Python version support planned for Sopel 8 will make testing on additional OSes less ridiculous (imagine if we tested six supported Python branches on two or even three platforms…)

Note that the original impetus (known problems on Windows like pyenchant) won't matter any more—we're ditching enchant and removing the spellcheck plugin from core entirely. But we definitely should at least test on Windows with the latest Python version, just for verification's sake.

[dgw]

We still don't have much motivation to worry about this, given the existence of WSL. Dropping to wishlist.