Build Image w/ wheel support

[jmadler]

Create a docker container image that retains multiple versions of python to enable building wheels and running tests across them natively. Configure TravisCI to use this image when building/testing.

This resolves #536, #411, #400

Known issues:

• lint doesn't work as expected on CI
• CI passes even if tests fail. This is a stopgap as 3.8 and 3.9 are still failing. Immediately following this landing, I will flip the error switch and make the build green by merging in the 3.8/3.9 fixes and addressing what remains.

[edschofield]

Looks good. Thanks, Jordan!