Baseline file changes / sorting

[Zeckie]

I've noticed that small changes in source code sometimes result in large changes to the baseline.json file (when run with --write-baseline).

It seems to be that the source files within the baseline aren't sorted, and sometimes the order changes.

Are the errors in the same source file written to the baseline in a specific order?

I think they should be sorted (both the keys of the json dict, using sort_keys=True, and the lists of errors)

https://github.com/KotlinIsland/basedmypy/blob/master/mypy/errors.py#L799

[KotlinIsland]

Implemented in #212