reliability: run CI for major supported python versions

[matt-codecov]

recently we pushed a version that worked if you installed it with a python version that has a wheel but broke if you had to install it from source. we should try to catch issues like that in CI

this is maybe obviated by https://github.com/codecov/codecov-cli/issues/286 which proposes providing wheels for more versions. we should look at how other python packages do CI and what platforms they choose to publish wheels for