Add CI pipeline, fix nearly all mypy errors

[mllewellyn]

Description

Okay, so this does a lot. I ask you yall to make many small PRs and here I am with a certified chonker. I could have split off adding tests, removing unum, making the repo pass mypy, and adding github actions but I'm limited on time so I'm gonna pull a do as I say, not as I do. This should not be a regular thing. I started with the goal of adding the CI pipeline, and this is what it took to get the repo so that CI pipeline could pass.

After this is merged we will be able to add the same CI pipeline to our robot repos without ignoring type errors from the toolkit.

Changes

• Add github actions based CI pipeline to run tests and type checking with pytest and mypy
• Remove plagiarized unum code. The code in robotpy_toolkit_7407.unum was straight copy and pasted from unum. This is not useful or in accordance with unum's license. This was also necessary to get type checking to pass.
• Fix type hints so mypy passes
• Reorganize test code so it is picked up by pytest

Tests

• pytest and mypy pass
• This still needs to be validated by running on a robot before it should be merged.