Bumpy mypy versions

[alanhdu]

Bumping the mypy version required re-ordering some of the overload.

Bumping the flake8 version created new errors, so I used https://github.com/ambv/black to autoformat the *.pyi stub files. (This is also helpful because it's what i used for the autogenerated stubs for #11). Is this the right approach, or should we just manually format these stub files?

[shoyer]

It looks like you may need to copy a few lines from NumPy's .travis.yml to check Python 3.7 working:
https://github.com/numpy/numpy/blob/4f1541e1cb68beb3049a21cbdec6e3d30c2afbbb/.travis.yml#L39-L41

[shoyer]

I do like the look of black formatted code here. It's not what we normally do for NumPy but I don't see any harm in trying it here. @sproshev any thoughts?

[max-sixty]

I do like the look of black formatted code here. It's not what we normally do for NumPy but I don't see any harm in trying it here

FYI we've used it in pandas-gbq, it's been great for not worrying about formatting. We've also rolled it out to our smaller / newer internal repos. Would be a mammoth diff for a project like xarray though

[shoyer]

Thanks!