Small improvements

[wchresta]

This merge request contains three small improvements:

• Clean up some unused imports
• Fix a typo in README (that I wasted more time on trying to find that I care to admit)
• Add unit tests for the README examples and mypy plugin

While testing I found some issues with the mypy plugin. I created the test cases but deactivated them.

The unit test for the README code could probably be refactored into re-using the code in the README. I'm probably not going to get to it for a while, so I might just remove the README unit-test if you consider this to be a blocker. We can add it later when I (or someone else) has time.

If you don't like the idea of the issue-style unit-tests, I can create a merge request just for the README fix, instead.

[jspahrsummers]

Thank you, this pull request is incredible! 🌟