Upgrade OS version

[dgrant]

Upgrade Docker OS from Ubuntu 16.04 to something more modern.

Related to #69 where there was a bug that does not appear in 3.5 but does in all versions >= 3.7.

Ubuntu 16.04 uses Python 3.5 which is quite old now. I don't think all users should be forced to use the docker image at all times. It should be possible to run unit tests or run the code itself on your own machine assuming you have all the dependencies set up, and most developers will have Python 3.7 and above.