Migrate Away From Codecov

Currently, we only use `codecov` to generate a badge. However, `codecov` can be unreliable for this and according to:

1. https://hynek.me/articles/ditch-codecov-python/
2. https://nedbatchelder.com/blog/202209/making_a_coverage_badge.html

We might be able to move away from `codecov` entirely 