What does this implement/fix? Explain your changes.

Enables embedded unit tests on AppVeyor

Does this close any currently open issues?

Shows failing tests on python3 during finalization but can't reproduce locally. It isn't consistent on AppVeyor either.

Any other comments?

Downgrades nunit to 2.6.4. It can deal better with exceptions that occur during finalize, unit3 just freezes and holds the ci worker until AppVeyor times it out.

Checklist

Check all those that are applicable and complete.

• Make sure to include one or more tests for your change
• If an enhancement PR, please create docs and at best an example
• Add yourself to AUTHORS
• Updated the CHANGELOG