Add Windows CI for cherry-picker

[Mariatta]

I think it will be good idea to add CI and test cherry-picker in windows environment.

But I don't know how to do that myself.

[Mariatta]

GitMate.io thinks possibly related issues are #113 (Questions about cherry-picker from Windows never-venv user), #88 (Make -h work for cherry-picker), #44 (Add the branch number to cherry pick commits?), https://github.com/python/core-workflow/issues/286 (Further automation for releasing cherry-picker.py), and python/devguide#292 (Add linkcheck to CI).

[webknjaz]

AppVeyor is a classical solution to this.
And now Travis CI has beta support of Windows jobs but I believe it's not that straightforward in usage.

[encukou]

See #311 for a Travis CI setup.

[webknjaz]

Thanks, Petr!

[Mariatta]

Closed in #311