snapshot based testing

[carueda]

In the spirit of project/development setup adjustments (complementary to #6) and for improved testing capabilities, it would be good to consider snapshot based testing, which I have used very successfully/productively in other projects, in concrete, using syrupy, which is a pytest plugin.

So, the idea here would be:

• if desired/needed, replace current unittest based tests with pytest (but probably not really necessary)
• add some basic snapshot tests to demonstrate functionality/benefits
• then add more tests and/or possibly morph some of the current ones into using snapshots

cc @danellecline

[carueda]

PR #18 (draft, initially)

[carueda]

closing per merged #18 - thanks!