TST: code coverage enhancements
We now have `codecov` [here](https://codecov.io/github/pydata/pandas) running on every build. You can get the browser extension to look at coverage in [github](https://github.com/codecov/browser-extension#codecov-extension)
- [x] make sure extension is working
- [ ] doc on how to use this / look at coverage
- [x] put additional excludes in `.coveragerc` to exclude things we don't care (#12721)
- [ ] increase coverage!
