Remove outside code coverage tools

[ajfriend]

As in #432, we've had issues with code coverage tools.

Instead:

• in the long term, get to 100% code coverage and stay there; PR-blocking test
• in the short term, hard code the current coverage number

[ajfriend]

Maybe we can separate out the currently failing tests so that the core library can have 100% coverage?

[ajfriend]

Yeah, and its causing headaches with cibuildwheel: #434 (comment)