adds coverage and failunder to pytest command

[fredshone]

Nice coverage - would be a shame if you ever let it slip...

Adds pytest-cov dependancy to default dependancies and "straps" to pytest command. So that running pytest also checks coverage.

I have added a fail for if coverage dips below 80%. This is easy to change.

[samgdotson]

@fredshone thanks for the pull request! Really above and beyond work. Embarrassingly, I forgot to actually add the coverage test in CI, despite my claims in #65. Regardless, your implementation in pyproject.toml is better than my approach, which was just to have it in the CI.yml.

More embarrassingly, despite appearances osier does not have 99% code coverage (one can dream)! The suggestions I made simply adjust the commands to

1. check only the osier source files
2. reduce the passing requirement such that this PR will pass the checks.

Once the changes are adopted, I'm happy to merge (I could accept the changes myself but I feel that decorum dictates I let you do that 😄). Let me know either way -- and thanks again for the PR!