Make wheels for Python 3.5

[codypiersall]

When updating CI to GitHub Actions, I noticed that Python 3.5 no longer works and builds properly. I think it's because Trio doesn't support Python 3.5 anymore.

It's a shame for pynng to not provide wheels for Python 3.5 just because of an optional integration with trio. We should some way or another. Here are some options:

• Use a requirements file, and load from it for CI. For building wheels this is probably my preference; there's no reason to always just use the latest of all dependencies, we just want to build some wheels.
• Slightly less preferable is to just skip the trio tests on 3.5. It would be slightly easier (less guesswork) than coming up with a definitely-working environment though.