Handle precision for fail-under option

[ionelmc]

No description provided.

[ionelmc]

@nedbat I am hoping I can use display_covered and should_fail_under from coverage.results without worries of them disappearing in a future coverage version.

[nedbat]

@nedbat I am hoping I can use display_covered and should_fail_under from coverage.results without worries of them disappearing in a future coverage version.

I'm not sure how best to make that promise. I'd be inclined to move them to a new public module. For now, know that things don't change much, and we'll hear about it if it breaks.