TST: code coverage enhancements

[jreback]

We now have codecov here running on every build. You can get the browser extension to look at coverage in github

• make sure extension is working
• doc on how to use this / look at coverage
• put additional excludes in .coveragerc to exclude things we don't care (Omit tests folders from coverage #12721)
• increase coverage!

[gliptak]

Where do you see this documented at? Thanks

[gliptak]

#12721