Add mypy as an optional code quality checker

[hillairet]

We have identified mypy as a good way to improve code quality in a project.
However, enforcing a type checker can be cumbersome for smaller projects, and therefore mypy should remain an option at least for now.

One big requirement is to guarantee than mypy returns no errors after a project has been generated because starting a project with many typing errors is not something anybody wants.
So maybe the best thing is to add mypy as an option with some warning at first, then fix all the typing errors before removing the warning.