What does this implement/fix? Explain your changes.

• Simplifies testing by giving the test DLL access to the internals of Python.Runtime
• Update NUnit to 3.5

Does this close any currently open issues?

Nope.

Any other comments?

Can be squashed.

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

All N/A.