Show source on mypy errors

[joelostblom]

What do you think about running mypy with --pretty? It displays the offending line from the source file, which I think makes it easier to troubleshoot. It also formats the error message in a similar way to what we like.

--pretty                    Use visually nicer output in error messages: Use
                            soft word wrap, show source code snippets, and
                            show error location markers (inverse: --no-pretty)

[lwjohnst86]

Ooo, that's very nice! Yes, make a PR with that suggestion 👍 🎨