Add appveyor.yml for Windows testing/wheel/sdist

[altendky]

For this to get used:

• Create an AppVeyor account and add this GitHub repository to it
• Settings I choose to change
  • General
    • Build version format
      • {build}
      • Note that with vcversioner or similar this can be overwritten to include the calculated version
  • Environment
    • Environment variables
      • APPVEYOR_RDP_PASSWORD
        • Whatever password you might want. You can click the lock icon to the right to mask the value. This would only be accessible to people you give permission to to change settings.
      • ENABLE_RDP
        • With my code leaving it empty disables it, setting to 1 enables

The RDP feature allows you to connect in with a full Windows desktop during the build and check files and try building stuff. This eases debugging of build issues. Hopefully this wouldn't need to get used much.

The rest of the configuration is here in the appveyor.yml file.

#41

[altendky]

For what it's worth, I personally use Azure Pipelines now and might consider GitHub actions. Both offer all three major operating systems and all python.org-supported Python versions in one CI system. Perhaps we want to wait for a PR for either of those systems, perhaps we want to take what we've got here to get Windows testing going now.

[bobfox]

Rather than use this for this single case, I would prefer we choose a single method to address all of the platforms. Are you ok with closing this and we create an issue for testing on all platforms?

[altendky]

Sure.