Restore testing of Windows Server 2016 on AppVeyor

[radarhere]

@nulano has suggested that we continue testing Windows Server 2016 - https://github.com/python-pillow/Pillow/issues/7910#issuecomment-2023624348

Does OS version matter?

We currently claim to support Visual Studio 2017, which is only available on AppVeyor using the Windows Server 2016 image; the GHA windows-2019 image only has Visual Studio 2019. I'm also hesitant about using older windows images on GHA after the (at least initially) short deprecation window of the windows-2016 image (#4188).

However, since AppVeyor doesn't install anything newer than Python 3.8 on Windows Server 2016, this is not entirely straightforward.

In this PR, I've installed 64-bit Python 3.9 through Chocolately.