show errors inline / run on change

[DetachHead]

might be tricky since mypy is so slow, but it would be cool if it worked like the typescript playground for example

[ymyzk]

As mypy provided dmypy, this idea may be technically possible. Another issue is the hosting cost of mypy Playground, the current server used for mypy-play.net is not good enough to keep mypy running or run mypy frequently.